It's more to do with movie length than file size.
You can have an exported movie of, say, 15 GB, but it is not this 15GB that ends up on the DVD.
The authoring program encodes the 15GB movie into something smaller.
If your movie is an hour long then this can easily be authored onto a DVD.( note; NOT AVCHD DVD )
A two hour movie would also fit, but the encoding bit rate would have to be less, resulting in reduced quality.
I should have mentioned in my first reply that AVCHD discs are limited to about 20 or 30 minutes as the capacity of a DVD is not as great as a Blu-ray disc.
So if your 50GB clip is around the 20/30 minute mark then this may well fit onto an AVCHD DVD.
If longer, then you need to burn to Blu-ray.
If you use FCPX for Blu-ray or AVCHD, then both of these are accessed from the Share >Blu-ray menu.
You would then select your internal burner for AVCHD and a connected Blu-ray burner for Blu-ray.