When you export a web tour from Strata Live 3D, using the Export > Web Tour menu item, several items are placed in the directory with the 3D model .JAR file.
One of these is a .HTML file, which includes an APPLET tag. To change the dimensions of the viewing window, just open the HTML file with a text editor like Notepad or Wordpad (not Microsoft Word).
Find the APPLET tag, and change the width= and height= parameters to whatever size you want.
Note that the larger the viewing window, the lower the interactive update rate will be.