These go down to a UK 5.5. A lot of conversions make the difference between UK and US a whole number, but in my experience it's less - I think US sizes tend to incorporate more "wiggle room". Either way, these are sprung trees so might squeeze into a slightly smaller shoe. If it's really tight, then it might be best only to leave them in for a day after wearing, rather than all the time.
I use these trees myself for a lot of my shoes, as they're crazy cheap. Unpolished trees are best for removing moisture, and cedar also deodorises your shoes, and probably repels moths from your wardrobe too! Worth a try for that price anyway.