While the image alt text is not content per se' it is still better to make your image alt text a sentence. The reason for this is simple. Search engines are not making keyword matches and never really have. They are using semantic scoring which is language based. A jumble of a few terms does not add the same value that a sentence does with a subject, predicate, and object. As well, since the image alt text is intended to be for accessibility, it is better to write a sentence for that reason alone.
I am not aware of any limit of the image alt text. I am sure there is one even if it is virtual in some way. Still, 100 characters may not be that bad. However, there should be a good succinct way to say what you need.
Remember that the image alt text benefits the image. Content that surrounds or is adjacent the image may benefits the image considerably, however, the image alt text does not benefit the content the same way. In fact, any effect is minimal. So stick with what describes the image and do not try and gain a SEO benefit for the content so much when there are several far more important signals.