Well, first of all I would think about what makes it look bigger… This can be lights, mirrors and a bright wall paint. As Mr. Kate says: create a mood board, it helps you to find out what colours and materials fit together ;). I would give the wall a new colour maybe a bright mint, but definitely a pastell tone (or white). Be careful with patterns – will make the wall look smaller, I guess. If you fix on some shelves next to the window you can put on lights and decorations that fits together with the rest of the room. (To many decoration will make the room look smaller but too less decoration will do it, either) What about pearl curtains? Just seen on Mr. Kates snapshat, in the nail art studio, could be something different than the usual material. Or use just one curtain in a bright and soft material and gather it up just on the right???
Just some ideas – hope you will get more inspirations 🙂