While developing a custom Outlook form, I found that the To: field would unexpectedly shrink to a few pixels tall and pull the remaining controls in the form upwards. The problem occurred when I added
Item.Recipients.Add "test" in my VBScript code to populate the To: field with a dummy e-mail address then clicked in the To: field. (I was using a dummy address to avoid accidentally sending test messages to unsuspecting recipients.) It seems as if putting the cursor into that field while Outlook is resolving the non-existent address (i.e. before it marks that address with a wavy red underline) causes the problem. The problem never happens if you populate the To: field manually with a dummy address, probably because Outlook is resolving the address as you type.
In my case, I found that typing Ctrl+K (Check Names) in the To: field restores the height of the field.
The solution during testing is to use a real address (such as your own).