I have a Squire Contemporary Series in HSS format. Ive tried almost all allen keys that may fit the hole by the neck, nothing fits, should I be worried? everything else works perfectly, in fact it plays well, just need to secure the truss rod for future adjustments.
Not every truss rod uses an Allen key for adjustment - for example, Taylor guitars need a special Phillips head screwdriver and a 1/4" nut driver.
But if your guitar has the original truss rod, some Squiers take a 3/16" Allen key. That's about 4.75mm, so metric keys aren't going to work. Others use metric. I'd suggest getting a set of keys and trying them out. Sets of keys are pretty cheap, and you'll find other uses for them.
Obviously 'almost all allen keys' hasn't yet included the right one! Next time you're in a guitar shop, buy the right one. But, as you say, no hurry. A lot of harm can (and has) been done by adjusting a truss rod that doesn't NEED adjusting!
If you can't find the right size of Allen key to adjust the truss rod of a guitar, a Torx key can be a useful alternative.
I just bought a Mexican Fender Jazzmaster, and the truss rod required a key that was larger than 4mm but smaller than 5mm, and apparently 4.5mm is not a size that is commonly found in hex keys. After a bit of experimentation, I found that a Torx T30 key fits perfectly.
(Converting Allen key sizes to Torx key sizes is not straightforward. Allen keys are measured between two sides of the hexagon, but the sizes of Torx keys listed on the Wikipedia page are measured point-to-point, and the naming system "T30" seems to be unrelated to the physical dimensions of the key.)
1) A Squire is not American and will use metric hex key.
2) Make sure there's no paint or debris in there. I just went through that with a Squire acoustic. I had to lightly tap it in.
3) Here's a message thread with exactly that problem.
"The walls of the adjuster socket are slightly flared -- the open end is slightly bigger to make it easier to get the wrench into the socket down in that blind hole. So the wrench only fits snuggly when it's seated all the way down in the socket.
Dust, polish, grime can build up in the socket so that the wrench can't be fully inserted. (Sometimes they even come that way from the factory, with finish overspray and polishing compound packed into the socket.) Take a thin pointy thing (like a small screwdriver or a straightened paper clip or something like that) and scrape the floor of the pocket while holding the guitar so that the socket is pointing downward (to let the debris fall out). That often fixes it.
If the guitar has had a previous owner (or someone else worked on it) it's possible they rounded out the socket a little by not sticking a tight fitting wrench all the way in. If that's the case, you'll need to get a 9/64" wrench and file it down (so it ends up a little bigger than 1/8"). That's very tedious because wrenches are made of hardened steel, but avoid using a belt sander or grinder because the heat generated may undo the hardening.
If you're tightening the rod, "helping" the adjuster can make it easier to turn so that you can safely use a slightly undersized wrench. Loosen the strings before tightening the rod, and/or clamp the neck/body between your knees (so that you're supporting the body/neck joint) and gently pull the head back with one hand while turning the wrench with the other hand. (People with older Rickenbackers are very familiar with that technique -- you can break the fingerboard loose if you try to straighten the neck on those just by tightening the dual rods without manually pulling the neck.)"
4) If all else fails or you want to call Fender at (800) 856-9801. They actually answer, no holding.