When it is in the web browser the size of the SWF is confined by the settings in the HTML.
Your Object code in the HTML will look like this;
Code:
<OBJECT CLASSID="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"
CODEBASE="http://active.macromedia.com/flash5/cabs/swflash.cab#version=7,0,0,0"
ID=navmovie WIDTH= 400 HEIGHT= 400>
<PARAM NAME="movie name" VALUE="URI of SWF">
<PARAM NAME="play" VALUE="false">
<PARAM NAME="quality" VALUE="high">
<EMBED NAME=movie name SRC=URI of SWF WIDTH=400 HEIGHT=400 PLAY=false
SWLIVECONNECT=true QUALITY=high>
</OBJECT>
The Heigth and Width are set in the example of above to 400. You can set it as a percent or as number in pixels if you so desire, some people desire to set it to 100% (WHich means the SWF takes up the whole webpage). KoolMoves automatically creates the values for you when you export as SWF and HTML.
From within another SWF the easiset way to control this is by loading in another SWF into a Movie Clip Target. There is a Import SWF as Object button which will do this for you. Basically instead of using LoadMovie use the import SWF as object and then adjust the bounding box. If it is still is not right then create a mask to trim of the portion you do not want.