In many PCB fav, panel without panelizing is actually cheaper. If you want 100 boards, they can panelize the order with other customers' orders for 200 or 300 boards. This allows you to undertake the setup costs of your order with other customers,and leaves production running for 100 units without intervention. Thus it is likely that the board house will charge you extra if you give them panelized boards.

There is no standard array size. But each PCBA shop will have a comfortable max size. If it surpassed that size, it becomes more difficult?to fit the panel into their assembly line. Another factor to consider is the stiffness of the array, which is influenced among other things by width, PCB thickness. The PCBA shops want the array to be stiff.

Actually there is no standard, you can ask companies A, B, C and D what they do. The largest will usually give the lowest unit cost however reduces ordering flexibility. You may want to use boards smaller than get the build/assembly optimum price in order to gain the ability to have smaller runs,smaller stock supply batches etc.