This is my Silk Road on the folder(not a very good photo, in the garage and taken with the phone).
The height at the front of the opening (behind the snout) is about 15cms (or half a foot in Bojo measures wot you're all meant to use). I think I measured 25 cms on the rails front to back. Side pockets are small but adequate for masks, flasks of gel hydroalccolique but pushed for an e-Trex) and the bottle pocket takes a light wind/showerproof and mostly has my cable lock. The inside will take my "sacoche" (frenchman's handbag) plus three big shopping bags folded up. This is quoted as the 11l size (large size I think; there is a 16l one which is too big to be really practical and which might be called extra-large). They have generally had sound reports from people on here but there are other makers in the market, Carradice as above, Zefal, Topeak etc. What's best value depends a bit on where you are and what is on promo (Carradice for me in France was much too expensive).
So far I'm pleased with it!
edit Forgot to mention that it does have a sort of stretchy bungy/cargonet affair on the top. I think the sack would have to be full before it became trustworthy. The sack is just long enough for a baguette "tradition" which is very useful if you take the bike to the boulangerie at the week-end (and would take a humongous quantity of pain au raisins)