Welcome to CT! Stick around and you'll learn a lot about doubled dies and true errors and everything else!
Your letters and numerals are normal size. You would be looking for moderate to heavy thickness of either or both along with some possible notching top and/or bottom. When in doubt check www.doubleddie.com to try and match up yours to Wexler's for any particular year. Please note though, not everyone (here) agrees with the new doubled die coins attribution with the single squeeze process. However, the experts like Wexler & Potter and others do agree.
Granted that machine doubling is not worth anything, I do collect them for humor, along side of my actual collection. I do not try to sell them as anything other then a penny or which ever the case maybe.
But I think as long as you label them for what they are and indicate they are for educational purposes, it would work. I've bought coins that were so labelled just for that reason.