Thanks, Justin and biesheuvel378713. I appreciate your replies.
I figured out a solution using the CSS Menu Writer wizard. I'll give an example below, which hopefully makes sense. Sorry for it being so wordy, but I wanted to make it understandable as possible for others needing a step-by-step process.
For my example, I created a three level, vertical menu where each level was a default value of 100 px width. I then wanted to shorten the Main level width from 100 px to 70 px, enlarge the Level-1 width from 100 px to 150 px, and no change to Level-2 width. (NOTE: I repeat some of the steps below for completeness.)
Bring up the CSS Menu Writer Wizard in the Edit Theme Menu window. Then enter the changes below:
Main level changes:
1) Select Main in Box tab
2) Under Dimensions, Margin & Padding (I'm shortening it's future reference to DM&P), select Item
3) Change Width from 100 px to 70 px
4) Select Apply to see change (i.e., shorter Main level boxes with a gap between Main and Level-1 boxes)
1) Select Level-1 in Box tab
2) Under DM&P, select Item
3) Change Width from 100 px to 150 px
4) Select Group within DM&P
5) [Now for the tricky part] Change Left Margin value to -30 px (negative value to create a negative offset) [Old programing knowledge coming into play.]
6) Select Apply to see change (i.e., gap gone between Main and Level-1 boxes, and Level-1 width now at 150 px, and now an overlap between Level-1 and Level-2 boxes)
1) Select Level-2 in Box tab
2) Select Group within DM&P
3) Change Left Margin value to 50 px (to create a positive offset)
4) Select Apply to see changes (i.e., overlap between Level-1 and Level-2 boxes
5) Select Finish to save corrections.
Bottom line: For my vertical menu, I had to change certain Left Margin values to create offsets in order to eliminate the gap; and as it turned out, to eliminate overlapping.