Looking at the FORM and PAGE Elements in Customizing ArcPad Help, I see no attribute for adding a vertical or horizontal scroll bars. Opening up a layer definition in ArcPad Studio and looking at implemented EDITFORM and PAGE objects, there is no option for scroll bars. Nearly as I can tell it is one of the limits of custom forms in that they have a static size. While you may see scroll bars and resizeable columns in the default editform that ArcPad produces on the fly, custom forms do not have this functionality. You will need to make more pages to fit all your content in the viewable form area.