The rule of thumb to find a quarter wavelength monopole antenna is using 234/frequency (in MHz). For 1/2 use 468/f and for 5/8 use 585/f. So a 1/4λ antenna on 2m is 234/145 = 1.61 feet and a 1/2λ is 468/145 = 3.22 feet. If you are making a dipole, then each half would be 1.61 feet and the overall length would be 3.22 feet, if you are making a 1/2λ vertical then the whip would be 3.22 feet. Keep in mind that only the 1/4λ monopole and 1/2λ dipole have the correct intrinsic feedpoint impedance to match a coax, the 1/2- and 5/8-wavelength antennas have much higher feedpoint impedances and will need to have a matching network to work. So if the antenna in your hand does not have a matching network, auto-transformer or something to match the coax impedance then the antenna can only work as a 1/4λ whip.
'91 Pickup - Imelda / '08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein