The five lengths
Write a tagline of about 60 characters, a one-sentence description of about 160 characters, a short paragraph of 50 words, a long description of 150 words and a list of five features. Almost every form fits one of these.
Save them in one document with the character count next to each. When a form has a limit of 140 characters, you trim the 160 version by two words.
What goes in the tagline
Say what the product does and for whom. "Invoice reminders for freelancers" tells an editor which category to file you under. "Get paid faster" could describe a bank.
Images
Keep a square logo at 512 pixels, a wide logo with a transparent background, and four screenshots at 1280 by 800. Name the files after the product so an editor who downloads them knows whose they are.