Is it possible to allow selecting wagons using ctrl+clicks/click with shift + click methods?
The Ctrl+click behaviour used in depots could be implemented. I don't know what Shift + click does, though (and I can't found any info about it at the tooltips either). This reminds me that I did not add any tooltips for the new widgets and behaviour.
Is it possilble to resize window when there are many cargos possible?
No, because in some cases it is impossible to display all the cargos at once. For example, check the refit window for the Ishizuchi Industrial Tram (HEQS) running with FIRS:
Tiny bug: create train with refittable wagon and empty engine, select an engine - there are listed cargos and refit is refitting whole train.
It is probably selecting the whole chain but not displaying the selection. Thank you for the report, I'll check it soon.
if you have a train with, say, one half open wagons and other half closed wagons, and you select e.g. grain as refit cargo, does it automatically unselect all closed wagons, which cannot be refit to grain?
In this case, the behaviour is unchanged from vanilla OpenTTD. You can select all options available to the current selection, and if you refit to a cargo option that is not available for all wagons of the selection, only the wagons that are able to refit to that cargo will be refitted. I agree that this is confusing, and because of that I would like to have a well defined behaviour to deal with this issue before implementing it. Have in mind that the creation of refit orders still uses the old method (and it is not the goal of this patch to implement partial refit orders), and both methods should stay as consistent as possible.