It is safe to say that Albino red belly pacus will grow to the same size as a normal red belly and grow at the same rate.
In fact, pictured below is my old Albino pacu named Wally. I purchased him with a regular red belly pacu at the same time. They both were quarter sized and were fed the same exact foods. The one difference I seen was that the albino was more docile and passive then the red belly. The red belly bossed him around and chased him which is why I rehomed the regular, but it is normal because the chances of seeing a albino red belly pacu in the Amazon are 1 in a 100.
I was able to keep this guy until he hit 1' then I had to rehome him sadly.