A matching device MUST be added between the antenna and the feedline if you wish to use coaxial. Adding a coil in series with the antenna at the base is one way of doing this. The cubical quad antenna is a four sided antenna, with each side about a quarter wave length long. Your quad's total length is feet Each of the 4 sides are feet. A matching device such as a coil in series with the antenna must be used if you wish to feed it with coax.
Most people are familiar with the centerfed, halfwave dipoles and inverted vees that are so popular.
There is no exact formula for calculating these antennas. The problem is that there are several factors that affect the antenna. Fortunately there is a starting formula for these types of antennas. MHZ Percent smaller for an Inverted Vee. Your dipole's total length is feet.