If the schedule still shows a Q400, the bins are quite small, comparable to what I've seen on the CRJ's. The area under the seat is actually larger. You could probably get a a larger roll aboard briefcase under there. Everything else is gate checked and DRO is pretty good about getting the cart around relatively quickly. The only time I've seen it go inside is in inclement weather.

If you have something larger and fragile, you can ask the FA if you can stow your item elsewhere. There is a baggage bin up front and some room in the back. However, this is up to captain's discretion and whatever commandments have come down from the mothership, so don't be surprised if your request is denied.