you can’t wear that dress unless you lose ten pounds first
That's not exactly an established idiom – at least, not in the word-for-word sense. However, at least in the US, it's not at all uncommon for women to specifically dress sizes when they are really talking about women's bodies.
For example, a recent health book is entitled Drop Two Sizes. In another book entitled Women Who Think Too Much, author Susan Nolen-Hoeksema encouraged women realistic personal goals:
Ease up on your goals so they become more reasonable. Try for a size 10 dress.
bare to each other the secrets of their souls
This simply means that the two people shared their innermost secrets.
Bare something to someone else means expose into the open so someone else can see (or know about) something. When talking about secrets, it's a phrasing you might see in a romantic setting. If I were writing a novel, I might say:
That night, in the moonlight, Jill bared her innermost secrets to Jack.
However, I probably wouldn't say:
The next day, at the construction site, Jack bared his innermost secret to Ernie.
Instead, assuming these two fellows carry metal lunchboxes to work, and drink their coffee black, I might say:
The next day, at the construction site, Jack let Ernie in on a secret.