You could solve this by just editing the tiapp.xml with maxs and mins closing the width and height. You can also define that the app doesn't maximize or resize:
<width>700</width>
<max-width>700</max-width>
<min-width>700</min-width>
<height>500</height>
<max-height>500</max-height>
<min-height>500</min-height>
<fullscreen>false</fullscreen>
<resizable>false</resizable>
<maximizable>false</maximizable>
<minimizable>true</minimizable>
http://tidesdk.multipart.net/docs/user-dev/generated/#!/guide/getting_started: