I'm not sure about the height. But for the width you can change it in the inc_header.asp file. I think the default width of the forum is already set to 95% with the exception of the very top of the forum. You could look for this around line 284:
"<table align=""center"" border=""0"" cellPadding=""0"" cellSpacing=""0"" width=""100%"">" & vbNewLine & _
Change it to your desired width.
You may also want to tweak these lines as well:
Around 314:
"<table align=""center"" border=""0"" cellPadding=""0"" cellSpacing=""0"" width=""95%"">" & vbNewLine & _
Around 328:
"<table align=""center"" border=""0"" cellPadding=""0"" cellSpacing=""0"" width=""95%"">" & vbNewLine & _
Around 430:
"<table align=""center"" border=""0"" cellPadding=""0"" cellSpacing=""0"" width=""95%"">" & vbNewLine