You are not limited to any particular slides size in Slidebank, so you can mix 16:9 'widescreen' slides with any other size of slide.
However, please note that PowerPoint itself can handle only slides that are all one size per PowerPoint file - it cannot mix differing sizes. Instead PowerPoint will shrink or stretch slide content to fit whichever size you set inside PowerPoint (Design / Slide Size).
Slidebank must therefore do the same when downloading a selection of slides having varying dimensions and it does this by making all the slides fit the size of the first slide you drag into a Selection (in the bottom window). So, drag a 16:9 slide into a selection first, and you'll find that all the slides in that selection will be 16:9 when they are eventually downloaded as a PPTX file.
Lastly, some Slidebank users place a dummy slide in the first position of a selection, simply to set the size of the slides when downloaded.
So, if your widescreen slides appear not to be formatted correctly, make sure the first slide selected is actually a 16:9 slide rather than a 4:3, or versa visa.