Simplicity especially in the 14 and 16 size can run very large...but some of the costume pieces do not run large. I have some of these I bought on sale one time, my daughter is into costumes....I bought some this paperlike interfacing I could see thru and traced the pattern out....then cut that looking at the other lines for a guide to go up 2 sizes. Next step is to make just the bodice out of muslin....old sheet or really just about anything (just in case your way off. If way off note where and why...too big, too small. Doing the trace and enlarge I took a discontinued pattern in an 8-10-12 up to an 18/20 and made a wedding dress for a good friend of mine who makes Anna Nicole look small breasted naturally! I did have to work with a sheet and couple of tries.....then with a marker, mark the seams, remove the stiching press, now you have a perfect patern, just make sure you add the seam allowances...this is time consuming if you are very different from the pattern....but once you have a couple of bodice patterns you can make all sorts of dresses....and they will fit beautifully. I also made her a bridesmaid dress once...perfect first try as I had saved the pattern from before and even though this one was fairly different, I knew where we needed darts, what width I needed. Do remember to allow on the sleave as well.....burned myself once or twice on that one. You can allow about 1/4" on the center frount for each size you need to go up....I tend if there is a zipper or buttons up the back to allow about an inch...makes it easier to put the zipper in. You would also allow about 1/4" for each size you need to go up on the center back in addition to that 1 inch. Another trick is to buy a pattern that is princess seamed as I imagine the costume one is in an appropriate size, it can help you get started in the right direction. PM if you have questions....I made my first dress when I was 7 yo....my mom wasn't going to get it done....so I snuck up to her sewing room and finished including a zipper....I have pictures...it actually turned out !
