I found this website because I wanted to take a thrifted (and very ugly) romper and turn it into a dress. I did the pockets backwards, made many mistakes, and did a very rough sewing job of this, but the end result was cute! if only it fit my daughter! (way too small)
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Congratulations, that looks very cute!
Measuring bodies seems to be the bane of every beginning sewist’s existence! Don’t feel bad it came out too small. You learned a lot and still sound excited, so try another project and let us know how it goes.
There are several threads on this forum about measuring. If none of them helps you, ask a specific question and someone will likely give you a clue as to how to do it better.
I recently sewed my first pair of Shale shorts, and it came out great except I sewed the waistband inside out. So I know how that feels! 
A couple possibilities about the size - did you hold the tape measure tight against soft body parts? did you add (enough of) a seam allowance?
That looks really cute.
As for it not fitting, if it’s woven fabric (as it appears to be) you’ll probably want some sort of closure on it, a button or a zip in the front or back to make it easier to get on and off. I made that mistake with my first dress for my daughter l she wore it twice before she could no longer get it on.
Another factor is the site generates patterns without seam allowance by default. So it’s likely you’ve generated it without seam allowance, then made it thinking the seam allowance is already included within the pattern. Which will make the finished garment too small for the intended wearer.