Just mask them and spray them. I don't have the information here but I have the exact dimensions for stripes at work. I can't remember exactly what the dimensions are right now but I can look them up tomorrow.
Edit ....
This Link on the FAA site is to a PDF file that shows the exact requirements for all markings on runways and taxiways.
For centerline markings, "The stripes are 120 feet (36 m) in length, and the gaps are 80 feet (24 m) in length. Adjustments to the length of the stripes and gaps, where necessary to accommodate the runway length, are made near the runway midpoint. The minimum width of the stripes is 36 inches (90 cm) for precision runways, 18 inches (45 cm) for nonprecision runways and 12 inches (30 cm) for visual runways. Centerline widths are not to be decreased even if all other marking sizes are proportioned narrower because of runway width."
For the side stripes, "The stripes have a minimum width of 36 inches (90 cm) for runways 100 feet (30 m) or wider and at least 18 inches (45 cm) on other runways. The stripes extend to the end of displaced threshold areas, which are used for takeoffs or rollouts."
Both the centerline and side stripes are white in color.
Be aware that there are frequently a lot of other markings on the runways, TDZ markings, aim point markings, numbers, and so on. All of them have specific dimensions, even the numerals. It all depends on what portion of the runway you are modeling.
To calculate the scale size, divide the dimensions given by your scale. For example, a 120 foot long stripe at 1/48 scale would be 120 / 48 = 2.5 feet long.