A portable grill is bought for where it goes rather than what it cooks, so the questions worth asking are about fuel supply, stability and whether it is actually allowed where you plan to use it.
Most portable gas grills run on disposable one-pound propane canisters, which are convenient and expensive per cook. Many accept an adapter hose to a standard twenty-pound tank, which is far cheaper for a long weekend. Check whether that adapter is supported by the manufacturer, because an unapproved regulator setup is a safety issue.
Tabletop models assume a table. Legged models vary a lot in how they cope with grass, sand or a picnic bench. This is the most common practical failure of a portable grill and it is not something the spec sheet addresses, so look at the foot design and the centre of gravity with the lid open.
Portable grills quote a small folded footprint, but the number that decides whether dinner works is the primary cooking area. Compare that figure to what you actually cook rather than to the packed dimensions.
Many apartment buildings, campsites and public parks restrict open-flame and charcoal grills specifically, and some permit only propane or electric. If the grill is for a balcony or a campsite, check the rule before the grill, because it may decide the fuel type for you.
Weight and lid latching matter more than they sound: a grill that does not latch shut spills grease in the boot of a car on the way home.
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It depends entirely on the building and local fire code, and the restriction is usually written by fuel type rather than by size. Charcoal is the most commonly prohibited, propane is often restricted, and electric is frequently the only permitted option. Check the lease and the local code before choosing.
Many can, using an adapter hose and the correct regulator, and it is far cheaper than disposable one-pound canisters for extended use. Confirm the manufacturer supports it for your model rather than assembling a generic setup, because regulator mismatches are a genuine hazard.
The larger ones are, for four people cooking in one or two batches. Compare the stated primary cooking area against roughly 72 square inches per person for a full meal, and remember that a portable grill usually has no warming rack to hold finished food.