The larger the slab panel, the greater the potential for random cracking. It all comes down to a choice between joints or cracks, and joints are definitely less expensive than cracks. And joints are relatively easy to maintain. And cracks are much more difficult and expensive to maintain than joints. Now lets talk about the panel shape.
Concrete shrinkage causes an even stress build-up across the slab.
Thus, a 15'x15' panel shrinks equally across all directions. But if you have a 15'x20' panel, the stress will be greater across the 20' dimension. It is quite likely a crack will occur dividing the 20' span into two 10' segments. You can reduce the effects of cracking by heavily reinforcing the slab. And reinforcing costs money.