The majority of the banners I make are 800 x 100. I have made 12 banners at 800 x 100 (or 110, 120, and 150), but only 4 banners over 900.
925 x 150 will probably work, but I wouldn't go over that. Anything over about 125 in height is getting kind of high for a banner, and when you get over 800 in length, you run into the possibility of problems if any users of your site are using 800 x 600 dimensions (although very few do nowadays, so that's why I say it will probably work).
Oh, and let me know if you need any help designing the banner.
