I lived in a 376 square feet (35 SQM) apartment. I wouldn't have wanted to share it, because of the noise/light issue (unless you and your partner have very similar sleeping patterns or are super sound sleepers.) For one person it was more than enough space, although the building design helped:
-large communal gardens including BBQ areas
-bike parking
-gym
-shared laundry
-clubhouse with pool table etc
-dumpster (put trash into a grocery bag and carried it out each day, so didn't need a large kitchen bin)
The shared facilities made entertaining easy, and bike parking helped a lot. I had a galley kitchen, where I once cooked for 20 (mexican party so it was simple food... I had to borrow the large pot of a friend), people sat on my desk, couch coffee table etc. We made it work.
I guess it depends on if you have hobbies that take up space or not. But for sleeping, cooking, eating (mine had a kitchen counter/breakfast bar, perfect for two), studying, watching TV, reading etc it was completely fine.
The only big "uncool" is having guests to stay, although a futon would help (I didn't have one).
As another posted mentioned: Ikea. The one near me has 35SQM, 50SQM and 70SQM spaces set up (the 70SQM is set up to comfortably home a family of four!) Amazing stuff!