You should look at several features:
- The type of battery. In this day and age, I would only choose Li-Ion products. A lead acid jump starter is either very heavy or won't start the car. Li-Ion in a compact form will provide enough current to help.
- What is the capacity in mWh or mAh of the battery pack? Bigger capacities are usually indication of a bigger ampacity, but this may not be always the case. The mAh capacity is usually a good number to compare, as there will be only one clear and definite number for this. Do note that sometimes manufacturers like to add up mAh numbers of cells in series, although this is strictly speaking incorrect. Watt-hour (Wh) or milliwatt-hour (mWh) values are more resistant against falsified claims.
- How many amperes is the product claimed to offer? Note these may be measured in different temperatures, so you can't always compare two brands based on this number.
- How long are the positive and negative leads? Some cars have the 12V battery in the trunk, and therefore, under the hood you have only one positive terminal for jump-starting. There may not be convenient negative ground locations very near the positive terminal, so be sure to purchase a product you can connect to your car easily.
Even four small Li-Ion batteries in a series connection should start a big engine, even a truck engine in fact. For example, a particular product with claimed 11000 mAh and 41 Wh weighs only 390 grams. Note it probably has three or four cells in series, and 11000 mAh is slightly confusing as one cell is 2750 mAh (if there are four cells), and they don't add up in that way. It is claimed to provide 400 A starting current and 800 A momentary current (whatever that means!), and claimed to start gasoline engines up to 5 liters of displacement.
I wouldn't choose a Li-Ion product weighing less than 150 grams. Even high-quality batteries lighterweight than this won't provide enough current.
Then you could consider some other factors, such as the ability to provide current to laptops, mobile phones, etc.
Edit: and an obligatory reminder. If you try to jump start a car with a failing battery, you may fry your electronics by using the jump starter. The reason is that the failing battery won't provide enough buffering for the electrical system, and the voltage control system of the alternator has been designed to work correctly only in case of a fully working battery. Thus, there may be a big voltage spike when disconnecting the jump starter, meaning expensive electronics will fry.
