I bought a 2 qt canton (80-201 on the link below) and I have never even come close to filling it up- in fact, I've never even had it 1/8 of the way filled up. In fact, since I installed it, I think I have almost stopped spilling coolant entirely. You'll find when you first get the motor installed and running, you will dump a little every time you shut it down, but once all the air is out of the system, you will reach a point when there is "balance" in the system and it will stop dumping coolant. If you overheated it would be good to have a larger reservoir, but if you overheated 2 qts wouldn't be enough anyway. Bottom line,1.25 qt is way more than enough.
http://www.cantonracingproducts.com/...tch_tanks.html