Changing resolution

I put about 5 videos from youtube on my phone, but only one worked because the others were too big in terms of resolution.  I already have a converter to get the format of the file correct, but my question is how to make the resolution of my videos from you tube smaller.  This is probably a really stupid question but everytime I search for a solution, I come across more converters.  I don't want to download another converter until I know that is what I need to do