Wow, you're learning fast. Seems like you've already got a good handle on brushes, props, clips, textures, detail, displacements, etc. See, hammer isn't so scary, it's just messy, unintuitive, outdated, and a generally nasty piece of software. But it doesn't bite...much.
I can't speak for the file size, though, since I save detailing for last.
You've still got some things to take care of, though. Most importantly, you need spawn doors. Here's a good guide for making the spawn room:
https://developer.valvesoftware.com/wiki/TF2/Creating_a_Spawnroom. I've done everything up to "Keep the bullies out" for you, but you should still read it all anyway so you understand basically how spawn rooms work, in case you need to rework the ones I gave you.
You've also got a nasty brush on the path leading to the upper point, from the red side. It's sorta sticking up out of the ground under the tunnel-house-thing, it's invisible from some angles, and players can get stuck on it. You also need an info_observer_point entity somewhere in the map, so players have a camera to look through when they're dead or just arrived to the map.
I've never played the instagib mod, so I'm not familiar with the gameplay balance surrounding that mod. As far as normal-style gameplay goes, there's a few other issues you might want to look into. Snipers standing on the upper point have a direct line of sight to the enemy spawn room, which is generally bad. Also, players have no real flanking route to the upper point. You could add a building inside the cliff walls that would give players alternate paths, so they could get to the upper point without having to climb the hill.
You're making good progress so far, though. Make sure you also get more advice from more experienced mappers, too, if you intend to take this map all the way to a finished state.