All of the trolling motors i have looked at are in thrust how can i determine how big of a motor will be needed for my 20' Renken

get the biggest you can afford    theres no other logical answer you can always use less power  but you can never have to much

I don't agree bigger is always better. A 65 - 70 lb thrust motor will move your boat very nicely under most conditions. Now, if you fish the Great Lakes or other large bodies of water where you can get into some serious winds, then you might want to look at something in the 80's. My boat is a 20' bass boat and pretty darn heavy loaded, I run a 68 lb thrust, fish a rather large lake and have never been in a wind that I couldn't still control the boat. This boat with this TM was also usded on the Citgo BASS Tournament trail for 1 1/2 years and never ran into a problem either

Another thing to consider, if you get much larger than 60 - 70 lbs of thrust in a 24 volt motor, you might need to consider upgrading your cables, unless you have #4 battery/marine cable already. #6 is the standard, but that standard was before these monster 24 volt motors came along. That or go to a 36 volt that will reduce the current draw.

THanks for the info this is my first boat you've been very helpful

If you are fishing a lot of "electric only" lakes (like me).  Get the BIGGEST you can afford.  Not only are they faster and your only form of motion, the larger motors have better staying power.  A 36v will last all day long with some kick still left after 8 hours.  A 24V...well, maybe a 12V, NO WAY.  

If you are NOT fishing "electric only" lakes, well then maybe you can consider a smaller motor.

I have an 18' aluminum with a 101 on it.  The Tm is my favorite and most impressive part of my boat.