I saw that, a little low quality

What would you say the source size shoukd be to make a decent x265? I sometimes encode on my own and I always use the best version I can find. However, sometimes some rare version arw only on certain streaming services with size approx of 20gigs (a movie of course). Is it good practice to make x265 out of 20gigs AVC if no better source is available? I usually try to make 5gigs result