The size and density of the ball is dependent on your ability to make it, there’s not really a lot of variance to play with as you use your Lao gung or palm points. Practice makes perfect.
To get more energy to build a qi ball, add qi cultivation. Breathe in cosmic qi, run it around the microcosmic orbit then down your arms to breathe out into the qi ball. Repeat and each breath makes it bigger. When it’s as large as a beach ball, compress it down to the size of a tennis balls and repeat. Carry on as long as you like then place it in lower Dan Tian to build your qi.