Karl, there are basically three shield types - the large, oval, Late Roman scuta; the large, round early Saxon shield; and a small, round Saxon 'buckler'.
The round shield is the same exact item supplied for both Arthurians and Saxons (or at least it is with Gripping Beast and Footsore. I'd be surprised if it wasn't for Saxon Miniatures too).
LBMS does glorious transfers for all three types, and these are scaled to exact size across all the main manufacturers - including Saxon Miniatures (e.g.
http://www.littlebigmenstudios.com/product-category/28mm-transfers/saxon-miniatures/arthurian-saxon-miniatures/)
The transfers come with a hole for the shield boss pre-punched, so you don't need to worry about that.
I just bought a selection of Footsore-sized transfer packs for the three different shield sizes, and intermixed them. They're all extremely well done.
(Edit: In fact, rather annoyingly, I notice he makes an additional pack of very nice mixed oval and round Arthurian shield decals which are available scaled for Saxon Miniatures, but not for Footsore for some reason...

)
The main issue I've found is that occasionally it's tricky to actually separate the transfer from the plastic film sheet once you've cut out the individual transfer. I have ruined several trying to get the sticky plastic film off the transfer and its backing paper.
My advice is to peel the plastic film right off the ENTIRE sheet of transfers once you're ready to start applying a few. Then LIGHTLY replace the plastic film before cutting out individual decals for application.
The other very definite problem is that the decals do actually shrink slightly within a few weeks of application. So if you've used paint to cunningly blend in the edges of the transfer with the shield rim, you will find a tiny white line appears right around the transfer a few weeks later. Which then needs to be retouched. After that, it seems to stabilise. Annoying, but not the end of the world...
The artwork on the transfers really is wonderful and brings these figures to life.
As you can see, on my figures I have not gone for steel shield rims. I've assumed that the shields are covered in stretched, painted fabric, and therefore extended the colours / patterns in each transfer, right over the shield rim. Weathering also covers up a multitude of sins

Cheers.
Richard