I am looking to put 2 half sheets(11x15) together . My question is if I do this what kind of box would I use to transport it in ? All the full sheets I find are 22x18. Help me please ?????

the 22x18 should do just fine. if you are butting up the 15" sides that measurement doesn't change so as long as it can fit a 22x15 it'll be ok, the board will just fix perfectly.

I have no idea... normally, if I make a cake that doesn't fit in a box, I'll lightly cover in saran wrap and transport without a box. Don't know if that helps you though, sorry...