What tin to bake a rectangular birthday cake for 25-30 ppl?

[PartyNovice]

My first time making a big birthday cake... It will be rectangle decorated with numberblocks. There will be about 25 kids at the party plus would be nice if there were some left for family but not essential. Slices will go in party bags.

Any idea how big a tin and then how you work out how much cake mixture to make?

A half sheet is 13x18 and yields 24 servings.
Full sheet is 18x26 and yields 48

If you don’t want to buy a new pan, I would bake 3 13x9 cakes and lay them side by side. Cover the seams with icing.

A full sheet cake, which would be the easiest, is the equivalent of 4 13x9 cakes.

Thanks, I assume that's inches! I was planning on biting the bullet and buying a tin, unless I have a roasting tin big enough...