What band or bands do you intend on using?
What coax or open ladder line feed do you intend on using?
Are you going to use an Transmitter tuner. Also known as an antenna tuner, but incorrectly designated.
The formula for a 1/2 wave dipole is 468 divided by the frequency in Mhz..
Leave a little extra to loop through insulators and pruning-tuning.
So if you're interested in 40 meters CW,,, that frequency band is 7.000 to 7.100 mhz.. You would want to center in the band.. so use 7.050 mhz
468 / 7.050 equals.. 66.38 feet.. each leg would be 33.19 feet.. Again add some for looping through the insulators. and adjustments needed because of near metal proximity.
You may want to read about Antennas in the ARRL antenna Handbook. You can find them at the Library...
Let me upload "Practical Antenna Handbook by Carr" to the files area here...
Larry W8LM
Larry Macionski