What do you think of these grow box sizes?

I'm planning to grow 2 plants and I'm wondering if this is big enough box. Width 4ft, depth 2ft, height 3.5 ft. Do you think plants will be very crammed in this box? Should I increase the height or width? Is there anything wrong if leaves start touching mylar foil on the walls?

This sounds like a dresser. You have 8sqft x 3.5 high. This is bad proportions. I have learned that to deal with heat and ventilation you need at least 5 ft high. 6 is better. If you are running low riders 3.5 would be ok, but you won't have enough space to run a carbon outtake at the top and covering 8 sqft with lights will be tough. Figure your filter and fan take 1 ft of height. Your light and reflector take 1 ft and you need probably 1 ft of clearance to your plants depending on your light source.

For two plants you could make a tent that is 28 inches wide x 16 deep x 6ft high. two 3 gallon pots fit in this space nicely. This is approximately 3 sqft. You could use a 150w hps all-in-one (cost $80). 1800 lumens. This is 6k lumens per sqft. If you like you can add a few day light cfls to boost your lumens a bit. You would likely need a 4" inline fan and carbon filter hung from the top and vented out as heat will likely be an issue unless you live in a cool climate.

I know all of this because I started in a 3.5' high box. I quickly outgrew it. Now I just use it for veg,

This is a good video on constructing your own tent. If you want something more stealthy then look for a kitchen pantry or wardrobe cabinet.

Hope this helps.