Language: C++

#include <bits/stdc++.h>
using namespace std;

#define INPUT freopen("input.txt", "r", stdin);

#define ll long long
#define ld long double

#define len(s) s.size()

#define fl(st, en, in) for (int i = st; i < en; i += in)
#define rfl(st, en, de) for (int i = st; i < en; i -= de)
#define fll fl(0, n, 1)

#define cy cout << "Yes" << endl;
#define cn cout << "No" << endl;
#define ci(s) cin >> s
#define co(s) cout << s << endl

#define vl vector<ll>
#define pb(x) push_back(x)

#define ms(arr, x) memset(arr, x, sizeof(arr))

#define inf INT_MAX
#define mx (ll)100000009
#define mod 1e9 + 7

//DSU

ll n, m;
ll parent[mx];
ll size[mx];

void make_set(ll v)
{
    parent[v] = v;
    size[v] = 1;
}

ll find_set(ll v)
{
    if (parent[v] == v)
        return v;
    return parent[v] = find_set(parent[v]);
}

void union_set(ll a, ll b)
{
    a = find_set(a);
    b = find_set(b);
    if (a != b)
    {
        if (size[a] < size[b])
        {
            swap(a, b);
        }
        parent[b] = a;
        size[a] += size[b];
    }
}
void solve()
{
    cin >> n >> m;

    fll
    {
        make_set(i + 1);
    }
    while (m--)
    {
        ll a, b;
        cin >> a >> b;
        union_set(a, b);
    }
    ll ans = 0;
    fll
    {
        ans = max(ans, size[i + 1]);
    }
    cout << ans << endl;
}

int main()
{
    INPUT;
    // int t;
    // ci(t);
    // while (t--)
    solve();
}

Compiler output:
a.cc: In function 'void make_set(long long int)':
a.cc:38:5: error: reference to 'size' is ambiguous
   38 |     size[v] = 1;
      |     ^~~~
In file included from /usr/include/c++/14/string:53,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~
a.cc: In function 'void union_set(long long int, long long int)':
a.cc:54:13: error: reference to 'size' is ambiguous
   54 |         if (size[a] < size[b])
      |             ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~
a.cc:54:23: error: reference to 'size' is ambiguous
   54 |         if (size[a] < size[b])
      |                       ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~
a.cc:59:9: error: reference to 'size' is ambiguous
   59 |         size[a] += size[b];
      |         ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~
a.cc:59:20: error: reference to 'size' is ambiguous
   59 |         size[a] += size[b];
      |                    ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~
a.cc: In function 'void solve()':
a.cc:79:24: error: reference to 'size' is ambiguous
   79 |         ans = max(ans, size[i + 1]);
      |                        ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:33:4: note:                 'long long int size [100000009]'
   33 | ll size[mx];
      |    ^~~~