Hi everybody!

I have finally decide to get proper road biking cloths and I am kind of stuck between sizes.
I am planning to get these shorts:
https://www.kbcyclewerks.com/store/c...-shorts-n300w/
I don't know If I should get small or medium shorts. My waist is 31 inches, but I don't want the shorts to be loose. I also need to get a jersey. Any suggestion???
I can't afford clipless pedals and shoes so I will have to keep on using my running shoes for a couple of months.(My feet hurt if I bike more than 70miles)