Opus supports dynamic resizing via Ctrl+Wheel in Details, Power and Thumbnails modes but not in Tiles mode at the moment. (Tile sizes can be set via Preferences / File Display Modes / Tiles, of course.) We may add it for Tiles mode in the future.
For M4V files, you may just need to add the extension to the Movie plugin via Preferences / Viewer / Viewer Plugins. Failing that, Enable/fix playback of various media formats in Opus has general suggestions on codecs etc. (If you do need more help with M4V playback, please start a separate thread.)