I use a half inch bit because time is money.
to get a bit that is long enough, I could only find a bit 4 or more inch long in a 1/2 inch shaft.
So i made the upgrade. (see pics)
To answer your question, if you are not going to go super deep you can use a 1/4 inch shank. I found with the foam and a smaller bit, the foam will wrap round the foam itself and mess up everything, So you will need to slow down your feed rate and lessen your stepover. But that is how i started. Stepping up white i Troubleshooted. It took me 1.5 years to figure it all out, but i started where you are, not know any lingo, or the ins and outs of cutting. I leaned on this forum when i hit a wall, every time having my problem solved that day. so i'm hear for you guys to lean on me. If anyone wants to make money doing this for friends, i'm happy to give you what i charge and the problems i found.