Abnormal EKG

Left atrial enlargement, AFAIK, and I'm not an expert at this, comes from fibrillation/flutter, or from a prolapsed mitral valve, the twin-lobed valve between the two left chambers. The left atrium enlarges when it is too active, such as during super-ventricular tachycardia, fibrillation, or flutter, and that is because it tries to do a better job of filling the chamber below it, the left ventricle, but unfortunately, it beats at the same time as the larger muscled ventricle. The ventricle blows up the atrium like a balloon when this happens, and eventually the inner lining of the atrium stretches, develops fibrosis (scarring) and that tends, in turn, to introduce fibrillation. Once you have fibrillation, it tends to come on more frequently, and longer in duration. That is to say, it is a progressive electrical disorder of the heart.

There are other abnormalities, though, such a premature atrial contractions, or PACs. These are the ones that produce the thumping in your chest wall, or what we call palpitations. Most electrophysiologists will ignore the odd PAC, say 200 an hour or so, but when they come on closer to 400 an hour, it becomes more 'burdensome' to the patient, but also to the heart itself. One's PAC burden can also cause some enlargement of the atrium when the burden is too high. At that point, the patient becomes worried, the EP gets concerned, and you'll be offered some sort of remedy, or at least an attempt at one, usually a form of ablation, perhaps a pacemaker.

Again, I'm just an interested reader still trying to sort it all out myself, so I really think you are best drawing up a list of pointed, and carefully considered, questions and flying them past a NP or the actual cardiologist.

I just had a Holter monitor test which said I had SVT, Can I exercise?

Probably, maybe...it depends on what your trigger is. If it's strenuous activity, I'm inclined to say no.
But you should have a device handy to measure your heart rate in the few minutes after you stop exercising to see if you remain in a high heart rate after your breathing has returned to normal, or within, say, three minutes of that time. If it's still above 100, you are in SVT, or perhaps fibrillating. A Kardia Mobile is only a few bucks, works really well, although it's not portable. A smart watch will do a good job. I use a Galaxy 4 for sweaty stuff, and a nicer, newer, Galaxy 6 watch. Apple has a watch, so does Garmin, and there must be others....Fibbit, etc.

This is a question that you should pose to your doctor.