I have had a few comments asking me what the measurements would be for different sized toddler clothing. While the overall answer is, I don't know, I have searched and found some info for you.
My daughter just turned two, but she is at the 90th percentile for height and 40th for weight. Keep that in mind when I give you the adjusted numbers here. I make everything to fit her (not by a set size). From clothing measurements by several companies, it appears that 2" is added to the length for every year. So, if a 2T is 21.5" long, then a 4T would be 25.5" long. I would also add 2" to the sleeve length. It also appears that you want to add about 3/4" for every year for the chest measurement (i.e width of the dress).
Here is what I have come up with for a 4T:
For the 4T, I would also make the armpit cut another 1/2 deep, so 6.5" instead of 6". I have added 1" additional fabric for the circumference of the arm. This might be a little excessive, but you can always cut it out if you need to.
For the reader who asked about 18 months sizing, you can just cut all of the adjustments listed above in half. Although, if it were me, I would just follow the 2T measurements because this dress is VERY forgiving size wise and the 2nd birthday will be here before you know it!
Please let me know how this works out. You can post any adjustments in the comments which would surely be helpful to other readers. Thanks!