What size mattress should I purchase?

I’m moving into my new apartment sometime this weekend (which kind of stinks because I paid a full month’s rent which started on the 1st, and it’ll be the 9th before I get in there), and I need to buy a mattress. I’ll probably get it from Ikea when I go to get a desk and chair.

The apartment is a studio, I’m estimating about 175 sq/ft on the Upper East Side in NYC. I plan to be working out of the space atleast 6 hours a day, so optimizing space is a priority.
There is a closet, but I don’t have the dimensions yet – I’ve considered purchasing a murphy bed apparatus and then using a standing closet for my clothing, etc.

Basically, if the murphy bed is an option, it will likely only fit a single/twin. I’d like a double, but that will take up more space, and would have to be on the floor.
I’ve also considered options like this, which I owned when living in Idaho. I’m not opposed, but I found couches don’t make the best beds in the long term.

Would you go with a twin or double, given the constraints, or do you have a crafty, space saving idea I should know about?