It depends a bit on whether you are trying to use the parameter within the same 'scope' or not. You could use Get(weight_percent); but that should only work within the scope of the same str phase. More generally you can define a new global parameter name and refer to that in your conditional statement. Something along the lines of the following should work...
str...
weight_percent !wtpc 50.003 (or whatever)
then
local size_a = If(wtpc < 10, some_param, 10);