how much big should the UTHs be?!

[severe_bomber]

The pic posted looks good imo. Your snake shouldn't have any problems with those temps. 69-72 is a bit cold, so adding a UTH on the cool side (and hooking up to a t-stat of course to keep the temp decent for a cool side) would be a good idea.

Absolutely. If you can control the outputs independently and have one cooler than the other then all is good.

My house is freezing so I use a infra-red bulb (on a dimming stat) at the cool end to keep the temperature above minimum. I think using a mat is probably even better though, as it won't kill your humidity like my bulb does!