Use it or lose it

If I masterbate once a week and stop doing PE will I lose any girth or length by not using that much or not?

No. Mothernature will only punish you if you ignore it for five years. If your under 30, ask again in 20 years.

Are you talking about PE gains?
If its cemented then not.
I would always do some very low key maintaining programm.

If you have only one errection per week you will loose size over some years. At least thats what older peopel who dont engage anymore experience.

I believe jelq and stretching rather than masturbation would preserve things much better.

Just jelq and stretch in the shower. Pretty easy I wouldn’t risk losing all the hard work you put into it.