I have used newborn on the TB and size 1 on ID who has dinnerplates. Hope Sh*tland never gets an abscess as would have to source premmie nappies I'd imagine!
You can cut nappies in half if needed but good scissors required.
I have been using size 4 pull up pants for my mare who took a size 7 in a woof wear hoof boot.
I prefer to have plenty big enough to tape up round the heel as I bought too small last time and found if she decided she wasn't going to comply with the wrapping processes and put her foot down it would slip off her toe and it would be a faff to reposition.
The pull up pants without the velcro tabs are more convenient I think, but if you've got smaller feet they might be far too big as I think they start at bigger sizes but not sure.
I went for size 2, up to 6kg I think? She has small cob/large pony size feet, not massive by any stretch but not teeny tiny and I'd say they were about right. Couldn't work out the sticky tabs on them though!
No idea if they'll stay put, it's been years since I last had to poultice (though this is for padding, not an abscess) and it was te side of the hill, getting dark and bloody cold, but it was worth a try I reckon.