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Just be guided by what is already in there, stick to a similar type of structure, length and wording and you have a better chance of the editing being minimal rather than a complete rewrite of what you entered.
If you think you're approaching a reasonable limit for a description, then go back and delete three-quarters of it, and you'll probably have something almost short enough to keep the editor from throwing it all out and starting from scratch.
added: You should also read the category description (far right in same header/cell as search box) as it will often give you some insight on what is acceptable in the category and they often point you in the right-direction of similar categories your site may better suited to.
And then assume not a word of it will survive the editing process. We'll make a silk purse out of your pig's ear, or sausage out of your prime loin chops -- useful or beautiful or not, you generally won't recognize the result.