There are basically two options - either check the ease settings, e.g. the back width can be wider in the case of the loose fit, or edit the size measurements in PRO mode. 

To edit the custom size, please go to https://www.sewist.com/editor/sizes, find the size you want to edit and click the pencil icon. Do not change the leading measurements (sz1, sz16, sz17, sz18, sz19, sz20) - but the others can be changed. E.g. the shoulder width is sz31, and the back width is sz47. 

You may also want to double check the spec sheet (it should be available for download on the pattern page) and see which measurements you'd need to change. 

Also how can these measurements be applied to the patterns I create using: https://www.sewist.com/editor/mix?

As for the mix and match designer - yes, these sizes will be available in your saved sizes, both on the mix result page and in Pro mode.

In addition to the issue with the sizes adjustment, I am trying to use the PRO mode to print a pattern however I keep getting an error 100: can not save item. Here is the pattern link: https://www.sewist.com/spl/#pattern/1-108-258-247-781-260-277-230-472-473-315-314-768-267-263-196-131-304-193-200-269-286-298/UNIVERSAL

Thank you so much for your help.

I tried editing shoulder width (sz31) (from 14cm to 13cm) on https://www.sewist.com/editor/sizes, without
changing any of the other sizes ,and click on the save button, but
after the process, the sz31 is  not saved, and the value always 14cm.

what can i do?

I tried again, and it seems that i can now change the value of measurement , as needed, in "my size" files.

thank you!

I am trying to use the PRO mode to print a pattern however I keep getting an error 100: can not save item

Order pattern #4692766