A 60x36" is a standard sized tub which you will easily find. 42" is also standard, but generally only for a soaking tub. You also need to be sure that the install will maintain the proper code required clearances to the adjacent fixtures. You can't crowd the toilet or vanity by going oversized here. Also, if this is a tub/shower combo, then it needs to have an integral tile flange. If it's a tub only, then you could do a drop in with an extended ledge on one side (or both). Or, there's no problem having a tub that doesn't fill the entire alcove depth. That makes the shower curtain hang better, actually and many were designed exactly like that from the beginning.