I don't believe there is a simple method in Dreamweaver.. But Math never hurt anyone..

1280/300 = 800/x

x = 187.5

You want the other at 187 or 188.. If you do it in CSS I believe you can do the decimal. Im not sure about that though..

I'd suggest changing the size in an editor rather than in DW.  Even though you change the size visually, it's still the same file.  Therefore the visitor has to download the full-sized image, even if it's made smaller.

I fell into that trap once.  I used DW's image resize to make some thumbnails without thinking.  Suddenly my bandwidth usage went through the roof.  That was before coming here and I plowed head first into the bandwidth limit with more than a week left in the month.

If you really need to do it inside DW and you want the proportions to stay the same, then by using the mouse, grab the lower right corner of the image, hold the shift key down and it'll keep the proportions correct.  I don't know how to do it by changing the values in Properties.