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.
Different guitar builders use different size truss rods and you need the specific type wrench and exact size for your guitar. Some truss rods are adjusted with a socket wrench while others use an allen wrench and others may use different type wrenches.
Under no circumstances should you ever attempt to use the wrong size truss rod wrench or you could strip the nut or socket and then you will never be able to adjust the truss rod again.
To find out what type and size wrench you need for your guitar, call Walden Guitars.
Walden Guitar USA information line (9am-5pm EST): 1-888-925-3369.
Then go to your local hardware store and purchase the specified wrench.
Also -unless your particular model Walden Guitar has an adjustable saddle, you will have probably have to sand the existing saddle down to lower the height should you choose to do so in an attempt to further lower your action.
Before attempting to file down your saddle by using fine grit sandpaper on a hard flat surface, obtain a spare saddle that is an exact match of your current saddle. If Walden won't sell you the part, try AllParts.com Guitar saddles on-line.
By having a spare, you can start over if you end up sanding it down too much. You can put a shim under it to raise it back, but it's better not to have a shim if you can avoid it.