How do you increase the thumbnail generated on attachment posts?

[I am David]

I would like to increase the size to something a lot larger than what is default.

This way users viewing the forum can see about 25% of the image and then if they wish to view the attachment in a larger view they can.

Thank you

 

[Paul B]

ACP search - thumbnail

 

[I am David]

ACP search - thumbnail

Thank you kind Sir!