You got your discus from a LFS and the guy who sold them to you said they would remain small, which tells me he knew they were stunted (stunting occurs from growing out young discus under poor water quality & conditions).
Not be too concerned here, because stunted discus can otherwise be healthy, and be kept healthy - if their color is good, they're moving around well & normally, their fins are outstretched, and they're eating well - that indicates they're healthy.
And even though they likely will not grow any larger than they are now, or very little, they can remain healthy if you care for them well.
No telling how old they are, but they're not likely babies, so they only need to be fed once or twice a day.
However, it's a real risk to their ongoing health by doing a w/c only once every 2 weeks. I suggest strongly that you re-up w/c's to at least twice a week minimum - 50% or more - and each time give the tank glass walls a wipe-down, and vacuum the substrate as best you can.
In my experience, if you neglect to do that, your discus will not likely remain healthy for very long.
Hope this helps you. Please take this advice.