Let's set the default buffer size to something that should fit most configurations

These appear to be the used rx cache sizes:

  - Marlin on AT90USB boards (e.g. Printrboard) => 63 bytes
  - Marlin anywhere else, using MarlinSerial implementation => 127 bytes
  - Repetier by default => 127 bytes
  - Repetier if EXTERNALSERIAL is defined in Configuration => 63 bytes

63 bytes should be pretty much safe in any case, user can increase that if falling into the 127 bytes group.