You will get many different answers on this one.
The best I can suggest is to match your pan to the size of cake board sold for each 'sheet'.
1/4 sheet - a 12x8 cake fits best on the board
1/2 sheet - a 12x16 cake fits best
full sheet - two 12x18 layers, laid side by side to be iced as one big cake 24x18.
NOTE: many places sell very flimsy boards. For full sheets one usually must use 2 or even 3 boards glued together to support the weight and size w/o the cake &/or icing cracking.
Many say even 1/2 sheets need to be on a doubled board.