MUltiple Pick List - Usability and scroll down

Multiple pick list needs to have a scroll. Imagine this: i want candidates do choose their nationality and, as you know, some candidates may have more than one (I can be American and French, at the same time). Selecting 2 options throw out 200 possibilities demands usability and a clean interface. I don´t want the form to look soooo BIIIIGGGG that we get candidates scary (or less motivated do fill it up). I thing that after 3 lines it should have a scroll down.