SylviaP is our local company so pretty much the main suppliers for leotards in our area. Her sizing is pretty standard Australian sizing, and Australian sizing is bigger than US sizing. What we call a size 10 is not comparable to what you would call a size 10 in the US. Our kids who wear a size 6 here, would be about a child’s medium in US brands.
But something I have found useful when ordering online from overseas is to email the company and ask them for the actual measurements of their leotards when laid flat. I ask for shoulder seam to crotch seam, and side seam to side seam across the middle of the leotard. If I am undecided between two particular sizes, I ask for the measurements of both.
Then (taking into account the fabric type, as some have more stretch than others) I can compare those measurements to leotards that currently fit to make a really informed guess about how they will fit.
In my experience, companies are more than happy to provide this information and do what they can to help you buy the right size. Especially when you mention that you need to get it right first time because postage from the other side of the world is too expensive for exchange to be a viable option.
I find this a lot more helpful than size guides, as girth measurements tend to lead to me buying leotards that turn out to be too short for my tall but very skinny child.