The sizes you mentioned are the internal dimensions of the oven cavity. The "quoted" sizes are a 30" convection oven and an 18" conventional oven. 30" is quoted size of a standard full size wall oven. You occasionally see a 36" but most houses have 30" or occasionally ~27". In all likelihood the large oven on the 48" is the same size you've been cooking in. The 18" is smaller but is good for roasted chicken, pork loin, beef tenderloin, casseroles, etc. Typically you can fit two of these in there at one time (i.e. a roast chicken and potatoes au gratin in a pyrex dish for example). Given the investment you'll be making on your range, it's best to look for trays, roasting pans, and other dishes that fit your oven well, especially for the 18" oven. There are plenty of options.

When I started my research for my kitchen remodel (about three months ago), my initial thoughts were strongly towards Thermador (for the range and refrigerator), and Wolf (I eliminated Viking due to quality concerns, and I hadn't yet heard of many others like BS, American, DCS, Aga, etc.). Then, I noted that Thermador doesn't offer all gas or open burners, and I better understood that my use of the oven didn't require the electronic functions (nor did I want the risk of having them burn out), and I thus eliminated Thermador even though I love the look (but function is much more important to me than style and looks) and it's star-burner shape does make it probably the best of the sealed burner options (ignoring output) IMO. If one wants to be a better cook, and understand the need of function over looks, then I can't see how one can pick Thermador, given the choice. I would pick BS or Wolf all gas instead. I think you might benefit from watching some of the Bluestar cooking videos Trevor has made (http://www.bluestar-ranges.com/Home_Page.html) - even though they mostly compare BS and Wolf, they serve as a nice educational source for learning more about what to look for in a range.

Don't know how well it applies to the Bluestar model that you are interested in, but I did a time vs. temperature plot on my Bluestar 30" RCS range right after it was installed (convection fan off): Not sure how representative this is for other models or how much oven burner adjustments at installation would affect the results (The air/gas ratio and thermostat set temperature are adjustable). It is just a single test on a single model sample. BTW, Bluestar RCS and RNB models have a single bottom burner. It is the Platinum model that has the rear wall burner without a bottom burner.

36" Wolf induction range has an electric oven. Most dual fuel ranges are gas cooktop with electric oven. Most people prefer an electric oven, unless you are doing specific types of cooking/baking that favor a gas oven.