I have a walden guitar which has a truss rod. It's action was high and I gave it to a music shop to adjust. They adjusted the neck but now the frets are buzzing a bit. Also I think that the action could have been adjusted by reducing the saddle height and I'm thinking of doing it myself.
Well, you have two choices I guess.
1) Use a pair of calipers to determine the precise size wrench needed for the specific guitar you own right now. Or find out through some other means (e.g., by contacting the manufacturer and asking what wrench size is needed).
2) Buy a multitool which will have the right size for now and also probably for any other future guitar you could ever own, and never have this problem again. here's one example of a guitar multi-tool, though probably a bit more specialized than you need.
Your guitar appears to be a nylon-strung classical guitar, and as such may not have a truss rod.
However, it's worth going on to answer the question in general:
Unless you have a very unusual guitar, a truss rod tool is either a hex key or a hex socket. These are common fittings in all kinds of fields, not just musical instruments: bikes, self-assembly furniture, etc.
Paying $7 for a tool you're not sure will fit, seems silly when discount shops (and eBay) frequently have socket sets, or bundles of hex tools, for cheaper.
The only slight issue with this approach, is that some sets are metric, some imperial. Worst case scenario, you get a cheap tool that's no use for your guitar but will be useful in other ways for the rest of your life!
The manufacturer should be able to tell you the right size, if Google can't.
Go down to your local hardware store and try various hex wrenches until you find the one that fits snugly. Then buy it there.