The weld symbol size is based on the font size of the weld symbol.
Tools -> Options -> Document Properties -> Drafting Standard -> Annotations -> Weld Symbols -> Text -> Font
This will control the symbol size, but will not control the length of the weld symbol line.
If you don't have very strict Drafting Standards you could use the JIS standard for Weld Symbols (Options/Document Settings/Annotations/ Weld Symbols). It's mostly the same as ANSI but is a much tighter fit to the font size, it just looks better.