I don't think it's that straightforward. I use WinXP SP3 also and have attached a screenshot with 4 File Open dialogs from different applications - Primo Ramdisk, BeebEM (an emulator of the 1980's BBC Micro), Audacity (an open-source sound processor) and Foxit (a PDF reader).
Aside from Primo Ramdisk, all the other File Open dialogs are resizable.
I've also attached a screenshot showing the window details (application, type, handle, etc) as reported by System Safety Monitor (security software that can control what applications are allowed to run and what actions they can take) with the entries for the above dialogs outlined. This shows they are all the same class (#32770) though a few other windows are also, which suggests that the dialogs may only differ by subtype. Another application I checked, IrfanView, uses the same class for its File Open dialogs but customises it more to include thumbnail and extra drop-down lists.