@ Girishwal , can you do the math ?  , I was never good in maths , had got 18 out of 20 in my last unit test  .

I stirred the pot expecting you'd search for it, but since you haven't, here's a starting point, which I've learnt from very experienced aeromodellers (not just fliers)

1. Understand what you wish your plane to do (fly level, toss around, do high alpha flights, 3d?)
2. Estimate a target weight that you wish to put in air (incl airframe, electronics, mechanical parts)
3. Have a rough estimate for how much power you need to make your plane fly as per 1 (trainer ~75w/kg, sports ~100w/kg, acrobat ~120w/kg, 3d ~150w/kg) - I am comfortable with these, to each his own empirical values
4. Divide by the avg voltage of your battery to get max current required.
5. Determine the wing area of the plane
6. Estimate a wing loading (works in tandem with 1)
7. Determine the stall speed of the plane
8. Multiply the speed by atleast a factor of 3 for those emergency situations
9. Plug in fluid motion formulae (with reynods number ~1) to arrive at approximate thrust required
10. Understand the pitch of your propeller to know how fast will your plane fly (convert that inch/rev to km/hr) and what RPM will be required
11. Know what is the propeller efficiency and what is the motor efficiency. multiply by Eta/100 to get actual power
12. Divide the RPM by the avg voltage of your battery to arrive at approximate Motor Kv.
13. Consult motor/prop specs that works in the ranges arrived at by 7, 10, 11 and ESC rating by 4
14. Repeat and double check

This is by no means exhaustive but helps get into air with some predictability. Each item above is a discipline in itself so here's something to ease out:

http://flbeagle.rchomepage.com/software/webocalc_1.7.6/webocalc.html

PS:
Here's something that will ensure you get more time for your math exams and a higher predictability of a 20/20 score

http://en.wikipedia.org/wiki/Indian_mathematics#Vedic_period

I can only get it till point no. 6 as it's far from what I have learned till now . Can you do the maths ?

I do feel you misunderstood my point...GhostRider has not mentioned how much is the weight he wants to put in air, the lighter the better, he mentions only wingspan... and we nowhere speak about AUW. I am not saying the plane will not fly with that setup. it might as well but that's more of a hit and trial...

And here is a sample calculation

Assuming
1. You need a trainer model (Kt = 0.75 - empirical)
2. Target weight = 1000g
3. I use about 75 w(75w/kg for level flying)
4. keeping in mind prop and motor efficiency (0.75 * 0.75) you need about 130W
5. avg lipo voltage on 3s is 10v, you need something that can do atleast 13A, in practice you will need about 20A for safety
6. The wing loading is (1000g/0.24sqm) about 4133g/sqm = 4.166 kg/sqm = 40.8 N/sqm
7. Lift = weight for level flight; Lift = 1/2 cl rho v2 s => v = sq((2 * Lift)/rho x cl); rho = 1.25, cl = 1 air; => v = 8 m/s (stall speed)
8. going in at about 3 times this value for max speed = 24 m/s, thrust is Newton weight * max speed * Kt (0.75) = 18 => max thrust = 18 * 24 ~ 432 N
9. Vmax being 24 m/s ~ 960 in/s = 57600 in/min; divide this by prop pitch for rpm required, on a 7 in ch prop approx 8000 rpm, divide this by avg voltage (10v on 3s) => ~800 Kv

The weight including electronics would not be more then 250grams.the original battery is 6v 150mah which weighs around 30gram.and 7.4 500mah lipo weighs 37grams.adding bigger battery will be better but this is a toy plane.it has brushed motors.if it was brushless I would suggest to use bigger battery. He has similer plane to this http://www.trendtimes.com/minircairplane.html

@ girishsarwal I have made 5 scratch build planes with just a hit on elects and all of them flew

@Kiranrc I guess you are right about the battery , then 7.4 v it is ( mah should be chosen by Ghostrider )

@ Kiranrc the link that you have given shows a plane which looks similar but wingspan , weight , length doesn't match the wingspan , weight , length which Ghostrider gave us.

Yes it has 6v motors, I think 2S 500mah 10c is enough and 5 g servos. I have flown this kind of plane with original setup without landing gear, with 1S 200mah lipo, the flying time was 5 minute.

@ayub he's using 9g servos

9 gram servo is heavy, I used 9 gram on my 50" sbach342 and a cessna 61"
anyway 9g can also work but this is a small plane, I think 18 or 20" span

Well I'm recommending 9g because I have used them in my smallest build which's wingspan is 20 inch and this plane's wingspan is 30 inch

And my build had 4 servos ( 2 aileron , 1 elevator and 1 rudder ) whereas this plane will need 2 servos only

Devangflyer 5 grams servo would be sufficient.9 grams may not be heavy but it should fit in planes fuselage.9 grams have bigger control arm.they take much space inside the fuselage.scratch built is different as u can place them anywhere.here u need to fit all the electronics in tiny space.

Okay Okay , didn't hink about that