Crossbody / Messenger Style Strap Possibilities
1. Theia strap:
From Marco's website: "With one unclicking of the lobster clip on the strap and rehooking it to the end circular hardware piece, the strap drop widens from 10.6 to 19.3 inches."
2. An additional leather piece that buckles to attach to the existing strap to lengthen it:

You may want to specify though what total length you want.
3. A adjustable slider / slide buckle:

This one I personally have not seen Marco do but that of course doesn't mean he hasn't done it or he can't do it.
Don't forget that you could use a leather puncher to add additional holes. I'm 5' 1/2", so sometimes I add 1-2 holes so the bag sits a bit higher for me. However, for MM's shoulder bags, I find they are just right for length for me. The Minerva short strap is perfect for me, though I have asked for rounded studs on that particular strap. The Hera strap is a work of art.
I do know though that some like the Soulmate to clear their elbow more easily when putting "her" on, so they ask for a longer strap drop for that style.
One more note on straps...personally I find the leather can make a difference. The pebbled, nappa, fiore, cuoio toscana, and tuscania are most comfortable for me. I imagine Verona would fall into this but somehow I've never tried a Verona strap. But weirdly, merinos has been a little slippy for me and vacchetta a little "hard." But, that's just my opinion.
Hope this helps!