The issue with high res. video files is fairly simple. It means the file you are trying to watch is most likely a blu ray composite. As such it will not play on any standard dvd player (and occassionally some computers).
You basically have two options: Reduce the size of your video frame - meaning physically shrinking the size of the video's parameters and not just setting your converter to mobile or mp4. I have used Video Dub to accomplish this as you can manually insert your own frame size as opposed to merely choosing a preset configuration which most converters have. You may have to shrink more than once if it doesnt take the first time so dont be discouraged.
Option 2: This is actually the easier but costlier method. You will need to purchase a high def dvd player that plays the highest res possible (currently 1080p). They are readily available online for only nominal cost and you will be able to play standard & hi-def (blu ray) quality discs. Just make sure the player is region code free (meaning it'll play ntsc or pal formats).
Again either of the above methods will work but bear in mind with reducing size in option #1 you will lose some of the video quality - so it depends how much of a stickler you are. And to save hard drive space I would move all vid files to disc or usb drive which reminds me... its also good if the dvd player you want to purchase has a usb port attached that way you have option of playing from disc or usb. Good luck.