Below are guide lengths but they rely on a good ground plane or counterpoise.
Suggested lengths for Bands:
70MHz (4 metres) 101.5cm
50MHz (6 Metres) 141.1cm
28MHz (10 Metres) 250cm
27MHz (11 Metres) 258.6cm
24MHz (12 Metres) 286.1cm
21MHz (15 Metres) 336.9cm
14MHz (20 Metres) 505.3cm