You are going to have a problem with this I am afraid.
The setting for making a checkbox multiline is ES_MULTILINE
and if you look here you will see that it states:
To create an edit control using the CreateWindow or CreateWindowEx function, specify the EDIT class, appropriate window style constants, and a combination of the following edit control styles. After the control has been created, these styles cannot be modified, except as noted.
So, it would seem to me that you have three ways forward, depending on what you feel is the best or most elegant for you.
Set your control in the resource editor as multiline anyway. Then it doesn't matter and will wrap. No need to have to change the setting.
Implement the needed functionality to limit the size the window can be reduced to. I can show you how if you are interested. This way, if you set the control resize properties correctly it can resize larger but only reduce down the a known dimension (ie: the dimensions you created it in the resource editor).
Possibly have two controls in the same place, one as multiline and one as single. And when you decide which you want to show, swap the visibility. But I think this is a bad idea, bit of a headache, and not worth the hassle.
IMHO I would do both ideas 1 and 2 and I would happily extend my answer to provide more information.
Update
Having looked at your images and the comments about translations then there is a fourth idea. If you use a third party application to manage the translations and use satellite DLL files then you can adjust the resources on a language by language basis. I sometimes have to make the default width for some windows wider due to their verbose nature.