You can harvest some now. Actually, if you snip off the top it will encourage the plant to create branches -- not all the leaves, just the topmost set.

Rule of thumb is to harvest no more than 1/3 of the living plant material - ie. you can't leave just sticks and stems. The plant needs enough leaves for photosynthesis. Some plants can tolerate more than 1/3 but start there. You'll get the idea pretty quick on how much to clip or not clip at one time.

FataMorgana

Look up 'pinching' basil. Maybe you can get a better idea or pictures of what to do with this size plant. Both previous posters gave wonderful advice. Basil grows a leaf pair, then the stem goes up in the middle. Pinching takes out that stem, encouraging the plant to bush out more, rather than just growing one stem. Personally, I would pinch some tiny new growth to encourage the bushing, then just enjoy the scent a little longer and wait before actually trying to harvest--give your plant more time and leave the larger leaves for now.

I would say that you sould let it to grow atleast three nodes . Then pinch off the last (above node)...continue with the branches the same way. Whe the plant gets REAL big, you can trim it more vigorously.