Ive been doing DWC for a while now and i have always started all my plants off in a 2 gal bucket for several reasons. For such a small plant a 5 gal bucket is a waster of water and nutrients. 2 gal is alot more user friendly and despite what people may think, the ph is very easy to control. You can use a 2 gal bucket for the first month and still have planty of room for the roots. After i switch to flower is when i move the plant to a 5 gal bucket. I actually grew a auto flower and kept it in a 2 gal for 2 months and only had to use a 5 gal for the last couple weeks. I ended up getting over 5 oz from that auto flower. One big reason a 2 gal is better for smaller plants is because the amount of oxygen being pumped into 2 gal is going to be more efficient then that of a 5 gal. The more air you can pump into your res the bigger and better your plants and buds will be. I have a commercial duty pump with 4 air outlets, i have 4 air stones in the two gal bucket and it looks like the bucket is erupting due to all the air in the bucket. It will make your plants grow faster and bigger for the first month if you use a 2 gal period.