While editing, clips can be sized in various ways. Right-click a clip in the bin (top-left) and Change Aspect Ratio.
Or perhaps use an effect to Letterbox or Crop to Aspect Ratio. The export dialog offers many A/R and resolution options...
The Resolution field... 
The Custom field... 
Upload a few clips, with info on how you want them to look on a specific device. Someone here will make recommendations.
Upload the file/s to a free server, such as Google Drive or MS OneDrive, right-click the link and Share or Copy. Copy the result and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, click SHARE | SHARE and change "restricted" to "anyone with link can view.**