2500 to 3000 words are required as per given in the paper of English essay. You must aim for 2500 words approx. it shouldn't be too short or too long .But I have seen people passing with 2000 words approx. It really depends on the quality of paper and it's relevancy to the topic that you are attempting. If you have written 3500 words but they are irrelevant to the topic , then for sure you will fail. Main thing is that you should right stuff that's to the point and relevant to the topic, not beating about the bush thing. And then vocabulary and language. Not flowering one is required. But atleast relevant to the topic. For example if you are attempting topic related to public ad then vocabulary will be different as compared to that of any scientific topic. And then your arguments matter a lot. Basically you provide these arguments in your outline. So that examiner can judge you from there. And last thing is grammar.
And yes thats true that you can only achieve excellence at these things by writing some stuff on daily basis. Just write atleast one page a day on any topic that you wish. And get it checked by any teacher if available. Or post it here at this forum. Hopefully there are many members who will help.

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''Sufficient for us is Allah , and He is the best Disposer of affairs" Al-Quran