The current World Cup had 12 groups of 4, for 64 teams you'd need 16 groups of 4. Each group has 6 games so you'd be adding 24 extra games to a tournament.
The group stage at this World Cup took 16 days. As the last round of group fixtures have to be played at the same time in each group, you'd probably only have to extend the tournament by 2-3 days. Four matches in one day should be doable- 12pm, 3pm, 6pm, 9pm kickoffs would seem reasonable enough.
I'd rather go back to 32 teams but, as that's unlikely, I'd have to say that 64 teams is better than 48.