I just had a look at the video, and I took start of the verse sequence. It's actually not that difficult to bring the chords and the strum pattern together, you just need to count it right. Usually you'll be changing chords right on the beat, so the down-stroke on the beat (on the number) is the first stroke to hit the new chord. Hence it's a good idea to make a little adjustment to the way I separated the two measures, just to make it easier to digest:
This: 1 & 2 &a3 & 4 | &a1 & 2 &a3 & 4
becomes this: 1 & 2 &a3 & 4 &a| 1 & 2 &a3 & 4
The way I wrote it would indicate "pushes" (chord changes made early), and that's probably a bit confusing for now.
If you have a "split measure", which means two chords in one measure (in this case both being played for two beats), you would play the pattern like this:
Chord sequence: C Dm Em F (2 beats each = 2 measures total)
C Dm |Em F
D d D duD d D du|D d D duD d D du
1 & 2 &a3 & 4 &a|1 & 2 &a3 & 4
So, when Justin speaks of "2 beats", it's the way I just wrote it down. If it's "1 measure", it's simply 4 beats.
One more thing: as a beginner (and even if you're not) you'll probably have a really hard time changing chords on a 16th note at mprmal tempo. It's okay to start the change a little early, in which case you're either strumming open strings or muted strings during the change (not deliberate, it just happens). The all-important thing is that the chord is in place on the beat. It's better to start early than to finish late. If your chord changes are (inadvertantly) trailing, it will be rather noticeable.