I usually put a 1/4 inch chunk in tanks with 1-2 plecos. The ones that have more plecos I add two pieces. I do this every other day or so. I find if I put larger pieces, the stuff starts to get gooey and mucks up the tank. All but one of my plecos are smaller 3-4 inches.
Check your nitrates - if they are okay and you are doing adequate water changes, you should be okay. Depending on what you have in with him/her, someone else may be nibbling at his veggies when you aren't looking
There's a thread that lists all sorts of veggies and fruits plecos love - watermelom, canteloupe, asparagus,...