Just as wattage is unimportant, size of your uro vs size of the tank is unimportant. The real question is can you maintain a safe ambient basking range(80-100) in the tank.
For 20 gallon long tanks the answer is (for the most part*) no.
While a 20 long seems big compared to a hatchling uro (and it is) it is almost impossible to maintain a healthy temperature gradient in that small of a space.
In my personal opinion a uro (of any size) in a 20 long is a health disaster waiting to happen.
Just like Robyn said temps are the important thing here.
*ok technically it is possible to maintain a 80-100 gradient in a 20 long it is just very difficult, time consuming, and prone to go wrong with the slightest change in ambient room temp. Unless you want a thermodynamic science project just stay away from 20 longs.
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Mike Wilson
mwilson@fuu.net