i have a modded traxxas 3.3 engine what is da biggest pinion gear that i can put on there i do not want to over gear it i have a one speed hpi rs4 2 and my spur gear is a 34

I'm not trying to be a grammar NAZI but is proper sentence structure and punctuation no longer taught in school?

Gearing for the engine will depend on how it was modded. Was it setup for more top end of bottom? What was done to the engine mod-wise? What pipe and manifold are you using?

it modded for top end and i have a ths pipe with a short manifold

I would be inclined to be prepared to try a couple/few different clutchbells to get the ratio that works best for the engine if you're staying with a 34t spur. Either that or go down on the spur gear and leave the CB as is. You likely will need to experiment here.

my racer 2 is a one speed i do not have no pinion gear yet im looking to found out what pinion gear should i start of with

So start with the stock one and see how it runs. Maybe get an extra one or two that each is up one tooth and try those. At some point the engine will run too slow to get on the pipe and you'll hear the engine running labored and probably too hot.