So my buddy is giving me a set of KREITLER alloy 4.5" rollers with a headwind fan. ( He owes me some money and can't pay it back.) The rub is that these are the 10" wide rollers instead of the 15" wide ones. My question is how much harder is it to learn to ride on these? I'll be riding both fixed and road on them.

You'll be fine. You're just going to be more likely to fall off (and it isn't nearly as bad as you think, trust me).

Started with the fixed-gear, it's a lot easier. It will carry you through the dead spots in your spin (which is the least stable part when you are on rollers).

You send those up here and I'll send you my nice wide Performance rollers.

No, seriously, you'll be fine. After your first few tries, 10" is plenty.

took a bit of getting used to for me, lots of falling off the first day or two. I mine are kreitler dynamytes, which are 10" and 2.225 diameter. I could definitely see an appeal to wider and larger rollers, but i suppose they will just make me a stronger rider.