Hello Hiroaki, happy new year, I have 2 comments about some possible enhancements to this plugin:
1.- It is possible to add custom names to custom sizes? It would be very useful, specially when we wish to build some functions based on the plugin. Those custom names would be uniques (a kind of "manual" ID) so we can query them directly.
2.- Another thing I want to mention is the redundant stuff. E.g. when we check the "Every posts" box, all following forms would be have disable that checkbox, so the user must to set and re-set all over the same form. In other words, is not illogical the presence of 2 or several forms with "Every posts" checked?.