This is a poem that I wanted to like because it's very cute and positive, a rarity when everything I see is all so mopey and dreary. The words are nice but the flow is all off and it makes it hard to enjoy when you're stopping in places you really shouldn't.
It's a shame because with some minor rearranging I think this could be really good. It made me smile but on a technical level it just kind of falls apart. Here, let me show you something.
The smile. Tugging at the corners of your little mouth. This feeling of joy that you have to let out. Whoever you are, never too tall nor too small. Wear a smile. One size fits all.
If you try reading it that way it has a more pleasant flow and all I did was make strategic use of the backspace key. Unfortunately the first three lines still read a bit awkwardly. With just a bit of altered word use it becomes even more readable, rhyming better and with superior rhythm. Look at this example with just a few, minor edits.
A smile. Tugging at the corners of your mouth. A feeling that you must let out. Whoever you are, never too tall nor too small. Wear a smile, one size fits all.
Reading it again I think you'll find it has a more elegant construction now and feels less wordy.
So am I right to assume you were inspired by MLP's "Smile, Smile, Smile," when you wrote this?