The SGPLOT "GRADLEGEND Statement" documentation has a link to "Optional arguments" The link incorrectly takes you to the optional arguments of the "KEYLEGEND Statement".
The SGPLOT GRADLEGEND Statement does not support the / VALUEATTRS= option. This could be a development oversight, or by choice -- who knows ?
The SGPLOT statements are interfaces that surface the very wide capabilities of the Graphics Template Language (GTL) and there are places such as this where some GTL functionality is NOT surfaced.
In your situation, you can drop down to GTL and write template code that SGRENDER uses to produce the same plot AND supports changing the size of legend values. The important GTL statements you will use are:
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