Essays should typically be structured like this:
10% of the word count towards your introduction
80% of the word count towards your body paragraphs
10% of the word count towards your conclusion
To the best of my knowledge there is no consensus about the size of your paragraphs. Often it is suggested that body paragraphs should be between 100-200 words but I feel that the true measure of paragraphs should be the idea that is expressed rather than stipulating a word count.
For me, the size of the assessments often dictate paragraphing: 1000 word assessment, Introduction + Conclusion of 100 words each and body paragraphs probably around 150 words; 2000 word assessment, Introduction + Conclusion of 200 words each and paragraphs probably around 250 words.
Whilst no hard and fast rule exists, what I will say is when you look at the overall essay all the body paragraphs should be a similar length rather than certain paragraphs being noticeably longer or shorter than others.
A key component to writing a cohesive essay is considering how the expressed ideas progress logically and sequentially to develop your overall idea or argument. Much of your planning should be directed towards ensuring that you present your argument clearly and convincingly ie which idea and paragraph should I introduce first, how does my argument progress depending upon how I've chosen to structure this essay and could I change this structure to strengthen my overall essay.