Depends on how inappropriate for the route it is, total demand on the route and your competition. If you're looking at 100 demand, 45 seats offered and no competition, you can probably fill the plane up until it's totally too small for the route. Too small for the route is a function of 1. Demand and 2. Average DEFAULT seat count in a fleet. Therefore, you as a player cannot alter how appropriate a type is.
Now if the type is 99% appropriate, yo'ull get the same too small warning as 1% appropriate, but the effects of tickets sold will be vastly different (i.e. at 99%, it's not a big deal, but at 1%, you'll fly empty).