One number leaves a lot out
A size such as 300 MB says how much storage a file occupies, not how it looks, sounds or fits your screen. Running time matters: the same storage budget spread over a much longer film usually leaves less data for each moment. Video compression and the number of audio tracks also influence the result.
Check the complete combination
Resolution, encoding, bitrate, container, audio language and subtitle tracks all help describe an edition. A device may recognize the container but not support the video encoding inside it. That is why changing a filename extension does not convert a movie into a compatible format.
For travel and limited storage
Use an offline option supported by the service carrying your edition. Check available storage, expiry rules and whether the chosen audio or subtitles are included. Test the downloaded edition before leaving a connection you can use. My List is helpful for planning, but it does not store the film itself.
Start with the MP4, MKV and 3GP guide for container terms, or read picture-quality advice when deciding between HD and a higher-resolution edition.