Pterygoplichthys pardalis is probably the most commonly sold albino common form which usually gets to around 14"
However there is also an albino form of Glyptoperichthys gibbiceps available which can get to around 20"
This is why you will find a whole tank of Common Plecos at most large Fish Stores and Local Aquariums as they very quickly outgrow the vast majority of tanks. They are beautiful looking fish if kept in the proper conditions but they really need a 6ft tank properly equipped to cope with large messy species. How they so often end up in a beginners 2 foot tank is testimont to the quality of advice available in many LFS.