I'm going to assume that you are going to build your own winterizing kit?
The suction hose is not very large about the size of the regular plumbing (~1/2"), nor does it need to be braided (it is a low volume, low pressure hose). To be honest, it would probably easier just to buy the kit with all the parts already there. Comes with the proper valve and the suction hose and all needed adapters:
You don't have to pay an exorbitant price from an RV supply house, but, even most of the websites (RV Wholesalers, Tweety's, Makarios, etc.) have them quite economically.