Does it keep rain water out of house, or just keep other thing?

It is designed to prevent the entry of rainwater and wind-blown garbage, snow and any other objects that would intrude into the home.

What are its outer dimensions? I want to know how big of a hole I need to make it fit on?

The caliber is precisely 4, but you can dig a hole slightly larger than 4 and you can install it precisely.

How it helps me to keep the wind out of my room？

Adding a ventilation fan, using a hose and air outlet connection to pull air out of the room and out through the roof cover, is our recommendation.

Can I use the flexible hose, Im afraid it will slip off and I have to buy an extra clamp to fix it?

This is the precise 4" air outlet, purchase the same caliber hose to install firmly. And use clamps to make it more. secure, this is a good way, you can also use tape wrap, I have seen people do this.

We usually have a few feet of snow in the winter, is this product appropriate and will it clog up from being snowed in?

It does not happen, the heat from the exhaust of the fan inside the house will melt the snow around it, I have really experienced.