You can set the initial size of suggestion lists, specified in number of items, by creating or editing the following DWORD value in your registry:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\InitialSuggestionBoxHeightInItems = 5
Default size is 5 if this value is not in the registry.
Note: a suggestion list will resize automatically when you scroll in it and additional suggestions are available.