Never been in an Inside on any of the ships, except the "old" Nieuw Amsterdam, many years ago, but if you go to HAL's website, click "On Board Our Ships", scroll down to "Deck Plans" and then across to Maasdam, you will get to see the various cabins.
I checked it out and it looks like Cabins K549 & K548 and Main - Deck 5 are 2 large Inside cabins. If you click on the cabin you can see a "virtual tour" of the cabin and it looks quite spacious with a full size sofa.
When you were planning on going on the Maasdam because if it's within the next few months, these great cabins may be gone by now.
Based on the deck plans and taking to HA, I booked 821 on A deck. It is 181 sq feet. I was on the Volendam in 1944 this winter and it was a very nice size. According to HA, it was 182 sq feet. Somehow we'll have to live for 25 days without that extra sq. foot:-)