Thanks for your answer. I noticed that in the case of leaves, you'd see 2 very small leaves at the top instead of a rosette shaped pattern like shown above. I'm not sure about this though.

Also, how long should the plant be before pinching off the top? And would it hurt if I pinch it an extra level below (at the second pair of leaves, number 2 in the picture below) instead of directly at the first pair beneath the bud (number 1)?

Basil is for eating! If you harvest and use it you will automatically be pinching it all the time. I start harvesting almost as soon as the plants have a couple of sets of leaves. And it does them no harm to pinch further down. New stems will grow very quickly.

totally agree

It won't hurt it to pinch it farther down.

Thanks a lot for the answers! One other question; does basil need any fertilizers often? If so, what NPK percentage would be good for it?

Yet another question:

Is it normal for the upper basil leaves to be quite small relative to the early bottom leaves? They seem to not get any bigger or at least grow VERY slowly.

Anyone?

The leaves on he flowering stems are usually smaller and more pointed than those on the main stems. Nothing to worry about. I'm taking a wild guess from what I can see out of your window that you are in the UK. (Architecture and colour of the sky!) If so your windowsill might not be quite bright enough for basil and it is just being a bit weedy because of that. You could feed with baby bio or similar plant gloop. And keep trimming those long stems and using them.

Haha, you were close. I'm from the Netherlands. I always bring the basils outside every day when it's sunny. So is it better to trim the main stem rather than the side branches to grow larger leaves?

Yes trim the main stem as well as the side stems.