What is the size of wrench for ovation guitar truss rod

Frankly, if you are unsure how to adjust your truss rod, take it to your nearest guitar repairman. You can cause a lot of harm to your guitar if you do it wrong. Ask the repairman if you can watch what he does so perhaps you will be comfortable trying it yourself the next time.

Edgar,
How are you determining that the neck is not straight? If you are thinking the neck is not straight because the strings are at different heights along the neck, the problem may be neck tilt or saddle and/or bridge height. You can raise or lower the bridge system or string saddles to add or subtract string height along the neck. In addition, neck tilt (particularly easy to adjust on bolt-on necks) refers to the angle that the guitar neck exits the neck pocket on the guitar body. This also effects string height.
Conversly, if you see a distict bow or bend in the neck when you look down the top edge of the neck, the truss rod may indeed be involved. Generally, most players like a little bit of "relief" in the neck. That is to say, a little bit of a concave bow. I prefer a relatively straight neck with very little relief. Just preference.
However, you mentioned you already adjusted the truss rod. Make sure the truss rod is functional. If you loosen the bolt or hex on the truss rod and the concave bow on the neck gets more pronounced, it is likely that you truss rod is ok. If you make adjustments on the bolt and there is little or no change on the neck relief, it is possible that your truss rod is not functioning or is broken. It is very possible to break a truss rod by over tightening but it would take some real effort to do. Still, it is possible.
Check out those points and see if it helps.
Confirm truss rod is working.
Check bridge height and saddle height.
See if adjusting neck tilt is possible on your model guitar.
Good luck.

Any inbus the right size will do the job. Every respectable guitarshop should be able to tell you what that is for free. Don't even think of using anything even a little smaller: it will ruin the truss rod's head.

First, get a mirror and strong flashlight and try to get a better look, maybe loosen the strings first to get better access. It is probably either an Allen key or hex head. Once you are confident you have seen it prpoerly, do something like this
http://www.ehow.com/how_5009065_adjust-neck-ovation-guitar.html

Takes a standard 'Allen Key' hexagonal wrench (I'm not sure of the size for your specific guitar but most are around 5 or 6mm)
Basic Epiphone manual here
http://www.epiphone.com/images/EpiOwnrsManul.pdf

Yes, a competent Luthier should be able to reset it... HOWEVER it may just need a truss rod adjustment. The truss rod bends the neck slightly to set string height in the middle of the neck.

Have you changed strings on it? Old strings get worn spots in them from rubbing on the frets that will cause these type of symptoms. If you put new strings on and get the same issue, check your neck relief (the amount of bow in the neck) to do this, hold your low E string at the first fret, and at the 12th or 14th fret. The neck should have a slight bow in the middle that puts it between 1/16" and 1/8" or so away from your string. If it does not have enough bow in it, you can adjust the truss rod. Most guitars use an allen wrench for this, it will be inside the soundhole on an acoustic usually, and on an electric it is normally on the headstock under a cover. There are many how-to's online about adjusting a truss rod, the key is to do it very slowly, only do about a 1/2 turn a day, retune, check it again the next day. Loosen the truss rod to put more neck relief, tighten to straighten the neck. If this still doesn't fix you up, let me know, and I'll tell you where to go from there.

The truss rod must be too slack then.
Is the sound ok? Does it play ok?
In it's current state the truss rod is doing nothing.
If the guitar is sounding ok and playing ok, then the next time you change the strings pull the truss rod out and throw it in the garbage.lol
Seriously though...just pull it out and put it aside.
If,however,the guitar is not sounding ok then try tightening it.
If you're unsure about how much to tighten it then I suggest taking it to your local music store for help with that.

No the truss rod is usually in more than that. Take it to a reliable Musice Store and they shold be able to adjust it for you.

Actually you can do this by adding shims under the saddle to raise the action. Then use the truss rod to make the action uniform if necessary. I try to avoid making adjustments to the neck because it could lead to undesirable perminent alteration to the neck. If you do make adjustments to the truss rod you should tune the guitar to pitch and let it sit left alone overnight to allow the wood time to adjust to the new adjustments.

Here is a link to an article on Adjusting action on an Acoustic Guitar.