Since the beginning of time I’ve been developing my apps with a base size of 320 x 460 and resizing at run time for bigger screens. Is there a ‘minimum’ size that we need to work to these days that is bigger than 320 x 460?
This site gives great informations regarding your question
I personally would optimize for bigger sizes than 320px to give an optimal layout to the majority rather than let the very vew smartphone dinosaurs dictate the market. But I guess with some extrawork you can make it great for both?!?
I use this 320px minimum size and resize as well. However, you can make a form somewhat larger than your minimum (320px) and Zoom down to fit the smaller phone. You get minimal distortion this way and then only on the smaller phones. But I generally avoid Zoom since I just make the smaller forms all 320px which seldom has object crowd control limitations.
I use centering and a maximum of 800px forms for larger screens and use a background appropriate to the app. Most of these larger forms (resizeable down to 320px) use grids or textarea objects that are easily resized to fit.
To see an example of a BS app using responsive controls see prograsa.com. To see a responsive app in jQuery Mobile that has manual re-positioning of controls see recipe.reciperasa.com. This app uses a minimum 320px for some forms and resizes up to 800px wide for others. These are both NSB web apps.