Pick a band or type a frequency, choose an antenna, and get the cutting lengths in metres and feet, drawn to scale next to a person.
Cut long, then trim. These are starting lengths using the usual formulas, which allow for wire and end effects. Height, nearby metal, insulation on the wire and the ground all shift the tuning, so cut a little long and trim while watching your SWR. Lowering the SWR dip means making the antenna shorter.
Formulas: half-wave dipole 142.6 ÷ MHz metres (468 ÷ MHz feet); quarter-wave vertical 71.3 ÷ MHz metres (234 ÷ MHz feet); full-wave loop 306.3 ÷ MHz metres (1005 ÷ MHz feet).