Create a parameter field at run time. You choose how you want to do it based on your deployment method. For this example I will just use a 1 and 2.
1=Width by Length
2= Length by Width
Assuming these are 2 different fields in you DB just create a formula field using this parameter.
if {?Parameter} =1 then {table.width} + " by " + {Table.length} else
if {?Parameter} =2 then {table.length} + " by " + {Table.width} else "ERROR"
The "ERROR" is not necessary but can be to catch items that for some reason didn't work correctly in the formula.
Change your group 1 to using this formula field to group on and it will swap the grouping based on the user selection at run time.