When you’re investing in a piece of luggage, the two most important things to keep in mind are:

Think small. A small suitcase (18 to 22 inches) will typically weigh less, and you’ll be less likely to get slapped with overweight-bag fees (applied to pieces 50 pounds and heavier). After all, “if you have a giant suitcase, you’ll most likely fill it up,” says Lisa Zaslow, the founder of Gotham Organizers, in New York City.

Consider the bag’s construction. “Choose function over high fashion,” says packing expert Susan Foster, author of Smart Packing for Today’s Traveler (Smart Travel Press, $20, www.amazon.com). The more structured a suitcase is, the better it will protect its contents — not just from the elements but also from wrinkles, as items will move around less.

After taking the bag’s size and sturdiness into account, look out for these features:

Ballistic nylon. A fabric originally designed for bulletproof vests. For durability, it’s generally considered to be top-of-the-line. DuPont’s Cordura nylon — so strong it’s also used for tires — is a good, affordable bet.

Ball-bearing wheels. The same type of high-precision wheels used for in-line skates, meaning they’re built to withstand rough treatment. They should be partially recessed to keep the bag stable as you pull. Recessed wheels are also less likely to get knocked off.

Carry-on strap. Allows you to piggyback a second, smaller bag on your wheeled suitcase.

Expandable gusset. A zippered portion of a suitcase that, when opened, gives the case up to three more inches.