It's a balancing act - you need enough detail so that someone who is familiar with testing but not with the thing being tested can do a complete test, but not so much detail that it becomes unwieldy.
For example, say there's 5 text boxes on the form. Each box could have 10 different cases - empty, too many characters, numbers, letters, punctuation, etc. Having 50 seperate cases for one form would be too much. Instead I might list the limits of each field and have 4 or 5 cases covering positive and negative scenarios with a note to repeat for each field.
Design the cases for someone who knows how to test but has limited knowlege of the product. Pay particular attention to any setup that might be required.