For smaller displays, it's less of an issue. But you should make a point to avoid 1366x768 resolution in a 15.6" display, and 1600x900 resolution in a 17.3" display, when it is reasonable to do so. Such size/resolution combinations make things onscreen appear rather large, and displays with these size/resolution combinations are usually low-tier LCD panels with very poor image quality due to low contrast.
But there is still a fair variety of ultrabooks available with 13.3" / smaller displays with resolutions higher than 1366x768 and/or above-average image quality.
Tell us your budget.