Graphic Size?

Hello all! :)

As a newbee here on the forum and webdesign.I find it difficult to customzise your own designs in Xara Webstyle(headings/logos etc.) when it comes to adjust the size.If you want to edit a size,you never get it right.Example you set a size to 855 pixels,and you ends up with 852 or 857.It looks like the graphic scale goes in steps by 5.
However it is possible to get it right,after trying XXX number of times.It's take shorter time to make the same design in a different graphic program.
Else I use Frontpage to make my pages,and find the use of navbar/buttons in Xara very handy.It works.