It's a 16"; as usual with all the bikes I find, I bought it because I thought it looked cool, not because of some boring reason like it would fit.
I am 5'10", but I have freakishly short 28" legs, so none of my trousers look good and I ride frames between 17"-21", depending on how old they are.
Pretty unique, though; never seen another 1980s Muddy Fox in such a small size. They predate compact geometry; their advice in the original advert for that bike (look in reader's bikes) was to pick a size 1" smaller than a correct touring bike measurement.