I did A level English many years ago. Best advice I got was roughly make it 1500 words. Craft your essay like this - 250 for the introduction and 250 for the summary.
Then you can put in 4 good paragraphs in the middle of 250 each. That gives you a good word count and forces you to think about structure as well.
Always, Always write a plan before you start. In my A level, I had to write 3 essays. I did two on the above formula and was really pushed for time at the end as I don't write quickly.
So I wrote a note to the examiner saying that my time was tight and copied my plan - expanding on the paragraphs in bullet points.
I got an A.
Also, writing a plan, means you don't have to worry about what you are going to write next, or panic about what youneed to write for the next question as you work on the current one.
If you think of extra stuff for another answer as you work, scribble it down on your plan. You'll be amazed how much time and worry this will save you!!
More tips:
Remember - 75% of the marks will be for what you actually write. The other 30% will be for spelling correctly, punctuation in the right places, having a good structure and flow - plus, not least, neat and legible handwriting.
You've no idea how many Brownie points you can score when the weary examiner comes across neat, well presented work after working through hundreds of messy, scrawled papers!
Good luck!