buy the most basic solitaire design... every hold hands don't tell me the guy won't feel the gf's finger how big if planning to buy right right?

personal experience, i played with my gf's fingers... check the size of the ring finger then compare it with my last finger cos guys usually have bigger hands
go to jewelery shop and try it myself
result was a pretty close fit

keep the receipt, you can do alterations to the ring if really different size