Before you size up a tree, size up the space it will occupy. “Ask, ‘What’s the height limit with a tree stand and an angel or star on top? What’s the maximum width to pass through the hall?’” says David Murbach, manager of the gardens division at New York City’s Rockefeller Center, where he has helped select the tree for 21 years. (Rule of thumb: Eight-foot ceilings fit a six- or seven-foot tree.) Next, consider species. Blue spruces have a pretty silvery hue but prickly needles. Among firs, the Douglas is traditional, but the Fraser and the noble, which hold their needles longer, are two of the most popular, says Gary Chastagner, a professor of plant pathology at Washington State University, in Puyallup. Avoid trees that have been lying in piles or in the sun; they can be dry and brittle, says Chastagner, who suggests giving them a gentle shake to be sure they aren’t dropping green needles. As Murbach points out, only artificial trees guarantee a pristine floor: “Just be prepared to miss that scent.”