Have the hose as big as you can without it being a sloppy fit and/or leaking.

You will be able to use slightly bigger internal diameter hose than that.

One tip though, cut all the plasitc hose off and use rubber from one connection to the other, its very hard to clamp rubber pipe to the plastic ones and not have it leak SHORT of squashing the plastic and therefore causing a restriction. I had to replace the very same hose on a friends series 1