^^^ Yes they could potentially double in size by harvest time!
Just at first glance I'd say they have a ways to go still. I'm guessing 2-4 weeks. The sure fire way to tell when they are ready to harvest is to use a strong magnifying glass/ hand held microscope/ jewellers loop, etc. to look at the trichromes on the buds. Once you have a good balance of clear/cloudy/amber trichs you're ready to harvest depending on your type of high preferences. I would wait until you get at least 25% amber trichs though, just don't let it go too long or the trichs will start to degrade.
-Mostly clear trichs = heady uppedy high.
-Cloudy trichs = ibetween a heady uppidy high and a couch lock high.
-Amber trichs = couch lock ripped off your ass wanna watch movies and eat cheetos high. =)
Peace! =)