If I am reading correctly this is not currently possible as you describe. Any calculation that references itself in a previous repeat results in a cyclic dependency error.
If this is not a public form, @DougBrowning worked on a javascript function for this.
Reading again, does the length and width measurement ever change from inspection to inspection? If not, why not ask it in the main part of the form, and then just bring those numbers into the repeats. If you format the calculation with the once() function, You could maintain if there were any changes to the length and width by looking at when the those were edited, i.e. you would have a log in the repeats.
The following posts, I believe has a description of the script as well as the best discussions of what has been tried and proposed work arounds.
Prepopulate answers in a repeat with last values - Esri Community
Have a repeat in survey123 default to last record ... - Esri Community
Idea you can support:
"Same as Last" option for repeat records in Survey... - Esri Community
