I think he meant "live performance?"
Based on what you're saying, I'd get a weighted keyboard of some sort. If it's basic and only has a few (or even no) sounds, that's probably all you need. I'd hold off on any kind of MIDI controller until you find a need for it in your workflow (e.g. "I wish I didn't have to use the mouse to automate this..."
The dimensions of the product should always be listed in the specifications. Get a tape measure and make sure before you buy anything. I always do this to avoid immense disappointment later. I've modded my desk along the way to be able to cram as much as i can and leave space for new things in the future. I'm pretty much always thinking about something i can get to fill a space on my desk.
In regards to getting an 88 key to put on the desk i would argue that it's excessive in any case. You're desk is also you workspace for pen and paper so you want some space on it! I'd suggest having a keyboard that large on a stand to the side so you can turn your chair to it when you need to. Get a wireless keyboard so you can sit it on top of the piano when you go to use it, set up key shortcuts for record, loop start, track start and undo so if you make a mistake during recording you can undo and try again without any need to move back to your computer screen. The only reason i could see to get an 88key is if you're looking to play a realistic piano or synth performance. It wouldn't really be of any benefit for orchestral arrangement (minus piano), since most instruments have quite a limited octave range, and you certainly won't need it for electronic music. On your desk have a 3 (maybe 4) octave keyboard controller with a bunch of assignable knobs for your soft plugins and leave the 88 key for anything that requires you to play right and left hand simulataneously more than 3/4 octaves apart. You'll find the capable remixers on here (as Timaeus has already shown) don't really need a large keyboard because of the vast possibilties MIDI programming offers.