How do I customize the perfect fit for my videos?

I'm trying to upload some anime to my Meizu SL 6, and is really a pain lol, it takes so long to separate subtitles and convert each one.

My current trouble is the zoom mode, with full screen im missing some areas to the left and right, with widescreen theres black bars, how do I find the perfect zoom size?

Well, if you stretched the picture to exactly 320*240 (the resolution of the M6's screen) then your video wouldn't be shown as it originally is. You could do that with virtualdub tho I guess... However I'd say black bars on the edges if I were you

which software do you use ?

Is it possible to get full screen with the Meizu Video Converter? Because there are a few settings that I can't change when I open the settings of the Meizu Converter.. like Width en Height