Couldnt you think of a better plant to use as a screen on your terrace...I believe Leylandii is not for this purpose.....They grow naturally about 3ft per year and can reach 100ft....so what will you do when they quickly fill the pot?...Im sure our friends here on GOY would have other suggestions which may help you...:>)

I don't know much about trees but I would take an un-educated guess knowing the ferocious habit of leylandi'i trees that pot grown ones would soon literally break out of them.

You really need to source something more suitable that is less invasive and visually more appealing.

I'm not sure how high you want your screen to be but if it is to be of any height at all you will need very large pots which would not be very portable. That suggests to me that one alternative might be to have trellis with climbers going through them instead. I would be tempted to try a pretty ivy - they are cheap - desparately easy to grow and fast to reach a reasonable height. One of the very large leaved varieties might be rather fun.