Signature sizes are limited by software and one of the limits is font size.
The forums standard font size is 2. You can bold it, color it, or even change the standard font to something fancy but not make it really big. Some fonts are inherently larger than others so you might experiment with this. Our Community rules have always proscribed signature limits, but a while ago the forum settings were change to enforce these limits after we had a lot of 'artistic' signatures that made threads somewhat cluttered to read.
It's likely that your sig predates this change.
You could do this perhaps:
WBCCI 12156 | AIR 3144 | WDC unit
1997 6.5 Diesel Chevy 2500 | Propane Powered Truck | Kayak Page
1991 34 ft. Excello +110,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles