Is A at the beginning of a sentence pronounced long A or short a?

It is normally a schwa (unstressed "uh" sound), but may be pronounced "ay" by the speaker, almost always for rhetorical effect. The noun or modifier it precedes may also be stressed.

"A cat is in the house."

"A cat, when indoors, may act differently than it does outdoors."

"While a cat would not threaten the bird population, ten cats would present a serious threat."

(meaning one, a single cat)