The only way I can really figure out if my PB sentences are working is to read the manuscript aloud. A lot. (And I don't mean reading it slowly to yourself and hearing the words in your head, I mean literally hearing the words.) Particularly if you've read aloud other picture books you like (and don't like), you will begin to hear what does and doesn't work for you. The issue won't be long or short sentences, but what kinds of sentences and punctuation force the oral reader to read, and allow the listener to hear, the sentences as you intend them to be cadenced. When sentences don't seem to be working, tapping the rhythm as you read aloud can give you a wonderful sense of how you can improve them, where you need more syllables or a pause before a word you want to emphasize.
Ann