A late 2014 MacBook Air uses a USB-A connector. You can use literally any external hard disk drive of sufficient capacity.
How much is sufficient: Time Machine requires enough space to retain an absolute minimum of one complete, restorable system backup plus sufficient space to finish a subsequent backup consisting of the aggregate amount of information changed from its previous backup, plus an additional amount of "overhead" that is not easily calculated for a number of reasons. Those factors conspire against providing an answer that is both simple and correct for everyone.
Here's what can help you decide: your MBA shipped from Apple with a maximum 512 GB of storage. If you were to use literally all of its capacity (which isn't really possible anyway), an external TM backup drive of 1 TB will be sufficient no matter what you do to fill that Mac to capacity. Considering the fact a 2 TB drive costs only a few dollars more, that's what I recommend getting.
Adding to that recommendation: I advocate having more than just one backup drive for the simple reason that any device can fail at any time. If you do something boneheaded like restoring things the wrong way (which isn't easy, but it happens) you could conceivably wipe out everything you have. So, whatever you decide upon getting make sure it's inexpensive enough to buy at least two of them. SSDs are nice, but speed is not important with Time Machine. Redundancy and simplicity are more important factors.