The select box takes the width of the largest option available. Ideally, you can reduce the length of the options or choose a more user-friendly UI interface to show these options to a user.
You can add this in your VF page to control the width but this is not something I would recommend if you are designing for mobile and tablets as well.
<style>
.multiSelectPicklistRow select {
max-width: 300px;
}
</style>
Simpler option:
<form method="post" action="/Tests/Post/">
<fieldset>
<legend>What is Your Favorite Pet?</legend>
<input type="checkbox" name="favorite_pet" value="Cats">Cats<br>
<input type="checkbox" name="favorite_pet" value="Dogs">Dogs<br>
<input type="checkbox" name="favorite_pet" value="Birds">Birds<br>
<br>
<input type="submit" value="Submit now" />
</fieldset>
</form>