as custom CSS in the RW Inspector (third icon from left > third tab).
Or in Configure (the spanner top left of the RW window) > Code > CSS.
But, as I say, that may not be the end of the thing: both Margin and Padding are additive in CSS. If you have a container 100px wide and add a margin of 10 on each side, you’ll be forcing it to 120 px (100 + 10 + 10). So you’d have to set the width to 80 in compensation; the same applies to Padding.
So it still worries me that you may be successfully changing one aspect of your object’s dimensions while other dimensions are either not accessible, not yet changed or otherwise unchangeable because there are more complex dependencies in the Cascading way CSS works.
Look out for the unexpected… typically jumping on the page, or wrapping.
What’s worse, of course, is that browsers tend to behave differently. IE actually tends to be more forgiving of containers wider than they should be.
You may well have to experiment; or you may be lucky!