In our own experience, LT masks generally size as follows:
| Bridle size | LT mask size |
| Shetlands, small/fine-headed ponies | X-small |
| Most pony-size | Small |
| Most cob-size | Medium |
| Larger/broader-headed cob-size, most full-size | Large |
| Larger/broader-headed full-size, Xfull-size | X-large |
This is Fenwick's own sizing guide which will give you more detailed information including key mask dimensions for each size