I found this Australian standard for you - though i do wonder if it should go up a bit with people getting taller and taller every year! "Australian Standard 3590.2-1990 states fixed height keyboard desks should be in the height range of 680 mm to 710mm" You could also test out your dining table to see if your desk chair fits under it at the right height.

I appreciate that you have used furniture that you like/have, is comfortable and within budget - you have created 2 separate functional areas in such a small space. Well done. I would try not to be offended by other comments because, if your brave enough to post photos and ask advice - you should be brave enough to take those comments with a pinch of salt... nothing in it. My honest opinion is... working with what you already have I would be inclined to bring the armchair forward a smidgen and squeeze a really tall artificial or real plant behind it. (Whatever your lighting allows for) Move the desk hard up against the right wall and position the lamp in the right corner (not middle of desk). I like the photo wall however would be concerned that 'seeing the frames' on both sides would make the room look smaller so I wouldn't position mirror on left side. Mirrors are great for that feeling of more space but not if the reflection is too busy in such a small space? I agree that you need height and in particular on that end wall. I think accentuating the end wall would be more beneficial space creating wise, than a mirror? Maybe even consider painting the desk / chair wall in a warm or cool colour depending on your taste! I think a small round jute rug would fit nicely sitting just under the foot stool? Have fun with it ...

Looking at your study photo, there is only one power outlet on the wall of windows. And this is stuck in the corner. I would recommend more than one power outlet above desk height. Then there is easy access to power for charging phones, laptops, plugging in computer screens, copy/scanners/printers. It can get dangerous if your using multiple socket power boards. If you are installing fixed cabinetry, it will be an issue to have to “get under the desk” every time something major needs plugging in. Cabinetry will need the holes on desk top for cables to then get to the one and only power outlet. That can be made easier by above desk height power outlets. Julie Herbert pictures show a lot of forward thinking has gone into position of power outlets and type of cabinetry.

I haven't seen a proper chair for the height of my table. May need to get a propert desk one day.So I occasionally stand instead of sitting.