You have to make it cosy without making it look overwhelmed- make it your happy place with hung floating artwork and bold jewel tones. Get some sleek furniture with bold designs. Another option is to choose a dark theme for your apartment to feel warm and cosy.
You can also add loads of mirrors to trick the eye into seeing the apartment as much more significant and brighter. If your apartment has a lower ceiling, go with low furniture and get creative with colours to feel cosy.
Another alternative is to create DIY spaces like tucking your retractable bed under a table or going for curtains over the doors to create privacy, as a door would take up a lot of space.