I have a 40 sized P-40 Warhawk and having a heck of a time getting it down the runway straight.
The main gear have 3 inch foam wheels,if i were to put 2 1/2 in rubber wheels will this make it a little easier to hold it straight
during takeoff?

Have you ever flown a tail dragger before? You need to use the rudder to keep it going straight down the runway. Going from 3" wheels down to 2-1/2" wheels won't help if you don't use the rudder. We have a couple of .40 size P-40's with 3" foam wheels at our field, and both track straight down the runway with proper rudder management.

See if you can toe the wheels in a bit, this helps tracking. The rest comes from just what was mentioned by Richard L..

Let it roll out slowly at first, don't be heavy with the throttle until it gets rolling and wants to come up on it's mains.

Either try to toe in the main wheels or have more elevator throw and maintain contact of the tail wheel with the ground.

Holding up elevator helps keep the tailwheel steering. However, if you hold up elevator too long, the plane may try to come off the ground before it has reached flying speed. Not Good!

The biggest help... SLOWLY advance the throttle. Use the whole runway. Lift off smoothly. it just looks nicer to do it that waay. (in addition to being easier to keep it going straight)

All of the above hits can help... the throttle hint is THE most important while taking off a "squirrley" taildragger.

Thanks guys for the suggestions,i have been flying this plane(exactly like richards pic) for a while now and haven't mastered it yet!
Most of the time i get rolling then it starts to swerving...i get nervous and want to "yank"it up off the runway and it really turns HARD to the left,i guess this is because of the low airspeed,one time i wasn't so lucky it went up about 6 feet and turned hard to the left and into the ground.
This is one of the p-40's i have and trying to get the takeoff down before i try my other one that is exactly the same and never has been flown,it has an OS 70 4 stroke in it 3 blade prop and really looks nice and don't want to practice with it.
As far as landing and flying this model i do pretty well.
I will try you're suggestions!
Thanks...
Oh one more thing,my other plane has Robart 615 retracts in her and when i roll it on the floor the gear cahtter,it really bounces like the wheels aren't square but the are perfectly round...any ideas on this one.

Test it by pushing on a flat surface without the engine running and the rudder neutral. If it doesn't roll straight then it will only be worse with prop torque. I once somehow bent the tail wheel strut on a tail-dragger off to one side and hadn't noticed. I had a day at the field where it was a real handful to take off, and had never been near that bad before. I needed to give 3/4 rudder to track straight and she wanted to snap roll on rotation. Turns out I had somehow bent the tailwheel strut to one side about 3/8". I found this by setting it on my cellar floor and pushing it to see how it tracked (I had toe-in right along with this model). Made way more difference than I would have thought, seeing how some tailwheels just caster. Now it's another thing on my pre-flight checklist.

As for me, it's not really important that the tailwheel is straight or the plane rolls straight. I play it by ear and use the rudder to keep the plane going straight down the runway. Once the tailwheel is off the ground, you will need to use the rudder anyway.

Here is a short video of my Kyosho Corsair taking off (1.9 MB):

[link]http://www.whitetyphoon.com/rc/kyosho_corsair/take_off.mpg[/link]

The tailwheel was on the ground for around 10-15 feet. This plane normally tracks to the left when rolling on all three wheels. Believe me, this plane is more squirley on the ground than the P-40 due to the shorter tail moment.

After complying with all the above advice, do this.
When setting up at the field, do so as far to the downwind end of the runway as practical. Begin the take-off from close in front of you and roll out at an angle toward the far end middle. This will keep you directly behind the plane and enable you to determine slight deviations immediately and make small corrections quickly - before things get away from you. And let that tail come up early. A nose-over is unlikely and better than a premature lift-off. You'll get it