Would think that a valve with a longer stem won't even fit. Now, if you got valves that were larger in diameter, then you can expect more power for some setups. Don't take that as a given though, hopefully someone who knows more about engines will say something soon.

When the posts speak of "oversize" valves they are talking about the diameter. I haven't seen a valve with longer or larger diameter stems, they just won't work so I think you are confused but on the right track for rebuilding your head. On valves go with oem or a good aftermarket brand name...Manley, Supertech etc.

Larger diameter valves will flow more if the bowls leading to the valve seats are ported properly. If they are not, it could be worse than leaving them alone. Larger valves also weigh slightly more than stock diameter, and could require higher valve spring tension to get them to seat at higher engine speeds. Its not so much at 6 or 7 thousand rpm, but once you get passed that you have to start considering things carefully. If you can swing it, go with one of the reputable venders on here. You won't find someone bad mouthing a Curt Brown or English Racing head. I have used Head Games in Princeton NJ, they work on some high end stuff and do nice work too.

Since you are not sure about the terms nor measurements you are speaking about, I will advise you move forward with extreme caution and do a bunch more research.

If you move to the +2mm valves, know you will have to change all the valve seats in your head casting, and that is not something you can DIY.

If you want +1mm valves, you will need a valve job and a trip to a decent machinist with good equipment and understands flow.

I would suggest you stay with the factory 6T valves, and get a set of single springs and do valve seals so you will have fewer issues.

You want diameter, not length.
Everyone knows that.

If you don't know that, its not worth the time and money. Some things won't make a difference unless they are actually calculated as such.

Sorry it doesn't say on the site weather it's talking about diameter or length. But it isn't clear what size is the oem size, they come in a variety of sizes before you add the +1 or +2. Are the oem sizes 30.5, 31, 32, 33 or 34.

Can you post a link to where you found those valves, that will make it easier to say if they will work or not.

Sure here it is. http://www.streettunedmotorsports.com/engine/dsm_engine_valvetrain.htm scroll past the springs and you'll see the valve stems I need to know the OEM diameter the only thing not available are rocker arms which I'm buying from another site

Standard stem size is 6.6mm. Std intake valve size is 34mm, +1 is 35mm, +2 is 36mm. Std exhaust valve size is 30.5mm, +1 is 31.5mm, +2 is 32.5mm.