Most modern day TV antennas are a combination of two types of antenna designes, one is the yagi the other is a log periodic.
The yagi gives it the gain the log periodic gives it the bandwidth needed to cover a wide frequency band.
In a yagi design the directors are equally spaced and mostly the same size after the first driven element.
In the log periodic the elements taper down (back to front) in a calculated length to cover a predetermined band. (or bands)
This is why some TV antennas taper down back to front, with the lower frequency elements at the back, and the higher frequency elements at the front.
A combo VHF/UHF TV antenna uses the lower frequency elements for the reflectors on the UHF band, and several same sized directors out in front for increasing the forward gain of the UHF section.
The VHF section relies on the UHF section to act as directors for its elements, now there are not 69 elements on the antenna as each element is sized to act as a 1/4, 1/2, 5/8 or full wavelength for what ever channel your TV set is tuned to, the internal TV tuner will automatically adjust it's self (AFT controller) to resonate the desired element to what ever channels your tuned to.
All the unused elements will try and act as either directors or reflectors.
So if the antenna your modding has a string of directors that are the same size then use the same sized ones to add to it, this will increase the forward gain of the antenna....To a point.
If they taper down then use the size of the last one in front to do the additional directors.
However your going to run into a point where the additional adding of directors will do nothing much for the gain as the added material could cause an RF blockage, this meaning that too much stuff is out in front and it's just not doing the job.
Also the additional supports needed to keep the added elements from sagging may cause unwanted problems, like causing the forward signal reception cone to droop to close to the ground, and you loose signals from further away.
This is why it's best to go with a co-phased array (two equal antennas) or quad phased array (four equal antennas) to add gain to an antenna system.