While there is a significant difference with regard to font sizes in Qt/Symbian vs Qt/Windows, I've never seen a difference in other widget sizes, as they are sized in pixels.
Re font sizes, on Qt/Symbian I've generally found that simply dividing point size in half gives pretty close to the correct results. Eg, a 14 point font on Windows should be replaced with a 7 point font on Symbian. I don't know of a way to manage this in the UI Designer or in CSS, but in Qt C++ you can use something like this:
I could see that, due to this font size difference, layout managers would make buttons larger on Symbian, in an attempt to make room for the larger font. Widgets with "hard coded" layout shouldn't be different, however.
QFont myFont("Arial Black", 8);
QFont myFont("Arial Black", 16);