I guess I'm only still commenting because you should make sure your players understand the intent, but it seems pretty clear: basically get 1 free casting of Enlarge Person and another spell (usable 1/long rest). In addition, if you have spellcasting ability, you can use those spells at their appropriate level.
If it wouldn't go too much against the usual grammar of 5e patterning, I recommend changing the first period in what I quoted to a semi-colon. It would more strongly tie together the first two sentences, making it clear the "this way" is the bonus spell usage, and making a harder divide between that and using existing spellcasting ability. Not necessary at all; it's just an idea to reinforce what is already stated.
If someone already knows Enlarge/Reduce Person, would they (in end results) simply gain a free use of the Enlarge version, plus learning another 1st-level spell.
I get what this means, but feel like it could be worded better. Unless there's a precedent with other feats that use similar language.The spellsí spellcasting ability is the ability increased by this feat.