Hi, I am installing a radiant floor system and will be using a Geothermal heat pump as my heat source.  I am sized for a 3 ton geo heat pump.  My question is how large of a buffer tank should I get.  There are 20 loops in total at 170' in length for each loop using 5/8" pex.  There are 2 zones one with 5 loops and the other with 15. 
Thanks in advance.

the geo mfg probably has a requirement to honor the warranty, so check with them.

to hit a ten minute runtime excluding your zone mass you probably will need 40 to 60 gallons at a 15 degree firing differential. however, without knowing what your differential or water temp targets is, that may or may not be the end of the story.

Would oversizing at say 80 gallons help to keep the temps ups since the geo is not a quick recovery type of heat source?

I have no idea what that means. I am not a fan of maintaining buffer tanks in any case, so I would say oversizing will slow response down, but who cares? as long as you have the output you need you'll be ok as long as you're patient during startup.

I guess I meant a lower temp heat source.

larger tanks will allow narrower delta T and thus lower max temp.

lets say you have a ten ton heat pump that is two five ton compressors in one cabinet. For your first five tone you could size at 10 gallons per ton and which would be 50 gallons. For the second stage five tons you could size for 5 gallons per ton which would be 25 gallons. Add them up and you have 75 gallons; so buy an 80 gallon tank and you should be fine. If you use the 10 gallons per ton for the whole tonnage of the units you would need a 100 gallon tank which will be more expensive and difficult to find than an 80 gallon.

Be sure that whatever size you go with it has large enough ports to support the required flow between it and the heat pump. Many people use water heaters as buffer tanks but the 3/4" ports may be too restrictive which can result in high pressures in the heat pump as the load is not being removed fast enough.