I have 8*4 prop and 2 quad copter motors of 1000kv & 650kv. which motor should i choose and also why

Welcome to the forum.
Lots of points involved in your question.

1. Kv means rpm/volt. You motors are 1000 and 650 rpm per volt.
2. This is with NO LOAD, ie, WITHOUT prop
3. So with a 3s lipo, say 12 volts, they should turn 12000 rpm / 7800 rpm
4. Assume 75% efficiency, so 9000 rpm / ~6000 rpm
5. How much load can they bear at these rpms? If overloaded, they run at lower efficiency and heat up
6. The load is the prop size. Both diameter and pitch.
7. Load bearing capacity depends on the POWER, ie, the rated WATTAGE
8. So the kv alone cannot tell you what size prop the motor can handle

Please tell us whether your motors are rated for 2s or 3s lipos, ie, for say 8v or 12v, and the Wattage, or max Amps, to proceed further...

If you don’t have this data, at least see how much the motors weigh. It’s then possible to guess estimate it’s power!

If even that is not possible, please post pics of both motors. We may recognise them!

So far this is to estimate what prop your motors can pull without overloading.
Suitability of the prop for the proposed use is a question to be addressed thereafter...

Hope this helps.
Regards
Iyer

I would like to add, If the Motor manufacturers name and size are known then prop size may be available in the Data Sheet of the Motor. This Data sheet may be found on the Internet.

This Video may help.