A small pump fits easily into a bag, but first check whether its head fits your bicycle’s valve. Presta and Schrader valves need a matching head or an adaptable connector. Try the pump on the actual wheel if possible: a stiff direct connector can strain a delicate Presta stem, while a flexible hose gives your hands more room.
Pump design matters too. A wider barrel moves more air per stroke and suits a higher-volume tire, but can demand more force at higher pressures. A narrower pump may feel easier as pressure rises, though it fills a wide tire more slowly. Choose for your tire’s working pressure range rather than the largest number printed on the package.
A gauge is useful when you need to know the pressure reached away from home. A folding foot or handle can make pumping easier, at the cost of extra weight. The useful travel pump is one you can carry securely and operate well enough to bring your own tire to its specified pressure.