You can usually find the answer to these basic questions with a little searching. No doubt there are plenty of other instances in this thread where this question has been asked and answered, but here is some insight from just the past 18hrs. Searching for "Tassel Sizing" or "Aberdeen Sizing" will likely yield further insight. Click "Search this thread" at the top of this page.
Help! Wanting to order BB shell tassel loafer. I have a sizing question. What size to order, me TTS 9.5D.
I have the following makeups
BB Unlined 9D Van last. Perfect fit.
Alden LHS lined 9D. fit is loose.
Being loafer they will give some. I just don't want to order a size and five years hence wished I'd gone down a half to my Van Barrie size. I have had many of these in my younger days but have forgotten the correct size. I have one BB sales agent who says go Van in the cordovan as it's larger than the calf makeup. NYC Madison dept., manager who says go TTS. What I don't want is to be racing for a plane and have to be fishing pant legging out of loafers.
I am TTS 9.5D and Barrie 9D but I wear a thin insole with all my laced shoes. For loafers I wear no insole and I hate heel slippage, so I go 9.5B for the Aberdeen and 9C/9B for the van. I have various shell tassels and full straps and also a tassel in Mocha kid suede, all in 9.5B. My suede model is the roomiest.