what stepper motor I need to get?

I am struggling with choose a stepper motor for my project.
what I am doing is controlling window blind using stepper motor.
so you can open and closed blind with some kind of switch to control the motor.
but the window is huge. so full weight will be quite heavy. probably 10kg-20kg.
any recommendation for a strong enough stepper motor for that weight would be really helpful.
many thanks

Are you going to run it direct, or through some sort of gear box? Or other mechanism?

What mount of force do you need to exert to move the blind?

If I can run direct without any extra part that would be really great. but if i can't using gear will be second option.

You still need an accurate measurement of how strong the motor needs to be. Assuming the blind is controlled by pulling on cords, you can use a hanging scale attached to the cord to see how much force is required to move the blind. If you don’t have a hanging scale then you could try attaching weights to it to get a measurement.

When high force is required a gear head motor with an encoder will be cheaper than an equivalently strong stepper motor. Also, a right angle gear motor (like a windshield wiper motor) would allow you to remove power from the motor and still hold the blinds in place; a stepper motor will spin freely (mostly) when power is removed. You will still need a motor driver for the wiper motor but it again will be much less expensive than a high power stepper driver.

Force is not relevant to a motor, torque is.

You've got to work out what turning radius you will use for the cord drum or sprocket or whatever before you can convert force into torque (torque = force x distance from axle of the line of action). You then have to allow for friction, 50% more perhaps?

Torque is measured in various units, Nm (newton-metre) is prefered, sometimes you see antedeluvian units like foot-lb, inch-oz, but they are all force x distance units.