dimensions for bowtie antenna

Did you do an Internet search?

There is no formula for bowtie dimensions that fits all applications. When you take some examples found on the Internet, you can scale them to the desired frequency.

The length depends on the taper angle and proximity of, for example, a ground plane (to get directivity).

By changing the taper angle (and dipole length), you have some control over the input impedance at resonance.

what about if i want to do the printed bow tie? how do i calculate the antenna? LxW of the antenna, not the bow tie.

If you have a design that is based on air, and you put it on PCB, the size must reduce somewhat to compensate for the reduces propagation speed because of the dielectric properties of the PCB material.

If the thickness of the dielectric is far below the size of the antenna (for example a 900 MHz bowtie on 1.5 mm FR4, you should think of a size reduction of some 5..10%. Of course there must be no ground plane on the other side of the PCB. You may know that vicinity of other materials (metal, plastic, ground plane of electronics adjacent to antenna) may require tweaking the size to get VSWR within accaptable level.