Seems like a big let down, missing a lot from v3.4 and can't even import v3.4 to v4.0, can't really be bothered to set up my 30 some window sizes again.
Yeah, I managed to work around it by forcing it to create a default sizer.xml in C:\USER\AppData\Roaming\Sizer\sizer.xml
and moving it to the program folder.. It only creates it if you actually change the default configuration or hit OK and Apply on any window size config. After that I edited using the find and replace all method. Stripping the unneeded options and keeping the mostly identical config names.
Old = <size description="640x480" height="480" width="640"/>
New = <resize description="640×400" height="400" width="640"/>
So basically going in the old v3.4 sizer.xml and renaming size to resize and just keeping description, width and height. Everything else is different in the v4.0 so I deleted the other options with find and replace. Then I neatly copy and paste all of my window sizes into the new v4.0 sizer.xml.
The default v4.0 config already has a bunch of 4:3, 16:9, 16:10, and common Youtube video sizes separated by groups which is nice.
The developer did a complete rewrite of the app so you should consider it as an early alpha preview. Not feature complete. Some buttons and menu's dont actually work and give out warnings saying so.