some questions about banner's creation

1) how to find out the size of that banner in photoshop?

right-click on the image, choose properties, you will see, among other things, the exact dimensions in pixels.

2) how to create such collage of photos on banner in photoshop? could you please answer step-by-step?

Open a new image in photoshop, choose a backgroundcolor (or transparency), make sure it has the right dimensions to accomodate all the images you want to put in.
Go to File - Place and browse for the files you want.
Scale them to the size you want.
If you have placed all the images you will have several layers, so open the layers-panel (if it isn't already). That's function-key F7. Grab the move-tool, select the layer you want to move and... well, move it. Grab your text-tool, add text.

You might also do a google earch for "photoshop collage tutorial", I'm sure you'll find some tutorials that explain it better.

Good luck.