Page Layout Control – Variable Width Based on Expression

Is there any way to set a control width to the result of an expression? For example, I’d like to be able to set a table width to something like "=Code.CalculateWidth(x, y, z)". When I try to do something like that, I get the error "Invalid expression. Literal expressions for this property must be of type 'length.’ Valid units are 'cm’, 'mm’, 'in’, 'pt’, and 'pc’." I’d be open to accomplishing this is other ways if someone has other ideas.