Size Of Thumbnails In Relation To Full Image?

I was wondering what you folks thought about this question, when it comes to creating thumbnails from full sized images for use in free sites, galleries, and members areas, do think there is a 'right' size of the thumb in relation to the size of the full image?

For example, on an image that is 1000px on the longest side, do you think the thumb should be 100px on the longest, 150px on the longest, 200px on the longest etc?

Do you think it matters as long as the thumbnail itself fits in well with the site it is being used on?

I dont think i have ever seen this discussed so wondered what you folks though, do you use a specific formula for creating thumbs?

Regards,

Lee