I wanted to know how does the increase the block size or rounds in AES is not a good idea
Just in general messing with well studied cryptographic standards is inadvisable. Especially if there is no clear benefit.
Adding rounds to AES is fairly safe, before it became AES the number of rounds in Rijndael was discussed. You need to make sure you continue to use a reasonable key schedule. This may have even some value, as we have seen attacks on reduced round AES making the safety margin smaller than originally believed.
Changing block size is far from trivial. It is not all obvious how to change AES block size while preserving it's desired properties. This is far more likely to end up in a weak slow cipher.