Mario is one of the best RPG series of all time. I say this without irony. SMRPG, the three PM games (four if you count SPM, but that's basically a platformer), and the three M&L games. All excellent. No duds in the bunch except... maybe Sticker Star? But even then I'm not sure.
But anyhow, Klyde, Thousand-Year Door is on the meaty side (~30 hours) and Superstar Saga is a little lighter (~20 hours). Both are excellent, but I would say that TTYD is heavier in dialog and world-building while SSS is heavier on crazy platforming trickiness. Both games are excellent in their own right, so I guess in the long run I'd recommend them both.
BTW, when I plan out what games I want to play and would like to know how long or short a game is, this is a pretty good resource: http://www.howlongtobeat.com/
. I'm weird and I plan out my backlog in advance. HLTB helps.