Someone already explained the difference between chien and chienne. But here's another tip: if you can't distinguish the noun's gender, look for other clues sorrounding it. In this particular sentence you don't year the second t in "petit". (It is silent, therefore masculine.) If you hear the second t in "petite" then the subject it's referring to is feminine.
If there are nouns or adjectives that have both a masculine and feminine form, then the feminine form will have a terminal
e where the masculine form does not:
But it is a myth that "feminine words end in
e and masculine words don't" because there are plenty of masculine words that end in
e, like "homme".
The truth is that there is no hard-and-fast rule for determining whether a noun is masculine or feminine based on how it's spelled. You just have to memorize a noun's gender as part of your vocabulary drills.
I think in "les" the "e" sound like if it was longer "leeeee" opening your mouth (the "s" is silent). In "le" you pronounce it with your mouth almost closed. Try using google translate and listen to both, "le" and "les" so that you can notice the difference. "la" sounds like the beginning of the english word "liar"
As of now I've noticed that articles are messed up here and there, and for this particular case answers "dogs are small" and "the dogs are small" should be equal, that apparently not. I understand the difference you've mentioned, but I would not still agree that second one cannot be an answer.