I know foxes that have chewed off live calfs feet in the pen so I don't think size is always the issue...
If your worried about foxes numbers post on some of the big shooting forums (pigeonwatch is one of the biggest - but biggest also means watch out for inexperienced. ukvarminting isn't so big but they generally know they stuff much more) and you may find people baying at your door to keep you clear of foxes! I would always just recommend interviewing before (the shooter and their lamp man if they use two people), getting references if you can, show them round the land and ask about safe shots (always with backstop, away from roads, footpaths etc), ask how often they will be there, ask what equipment they'll be using (minimum - rifle calibre of .222 or larger unless specific situations determine otherwise), ask to be texted prior to coming on your land and whether they'll be using a vehicle (if you don't mind). They are likely ask for signed permission though, don't be put off by this, it's often a legal requirement by the local police.
Remember though, fox management, not fox inhilalation is what you want!! A healthy fox generally stays away from humans and their animals. If the shooter has a trail camera ask to see if they can use it on your land to assess numbers.