hey budman,
The pot size should be from one gallon or larger if you wont want to have to transplant more than once. You will want to avoid transplanting too many times as this will stress the plant. As far as mericle grow goes dont use their fertilizers. Get their 3 months feeding potting mix or organic choice soil and mix that with the same amount of peat moss or coco coir. You can find the soils and peatmoss at any walmart, lowes, home depot or gardening store near you. Coco coir can always be found at your local pet store. Just ask for it and im sure they will help you out. Water the soil well and note how heavy the pot is so u know when it has plenty of water and when its dry because it will weigh alot less. This soil mix should allow you to water about once a week it it hasnt rained during that time.
When they are a month old and have around 3 or 4 nodes (node being a point where the plant is growing out from the main stem) you should start adding nutrients. You dont have to spend a bunch of money on nutrients though. A manure tea or kelp extract works fine and is cheap. Water with this solution every other watering. About once or twice every two weeks. You can go without nutrients at all during the life of the plant but it will deff. give you a better finish in the end. One last note, now is a good time to start. Just find a place where you can give your plant direct sunlight for 8+ hours a day.
Good luck with your grow,
Drfting07