If your coax velocity factor is .84 and you want your 1/2 wave multiples/antenna centred on 27.185 mhz then your looking at 15.2feet or 15' 2 1/2 inches.
if you want to repeat the antenna swr to the meter,then repeat that swr back to the amp,s output,and also repeat the amps input impedance back to the radio the safest policy is to use 3x15' 21/2" leads.the meter isn't a load its a pass through device so shouldn't affect your swr readings,
the linear input is a load so
if after that the swr is high at the radio then the input swr of the amp is poor,due to poor design of the amp.a good amp should have variable input tuning,most cb transistor ones don't.
you could mess around with the lead between the amp and radio to fix that but it won't have any effect on antenna swr,only what the radio see's,if the radio has high swr protection circuits it will cut power output if it doesn't see a good swr,another way you could do it is keep the leads the same size and use a matcher at the radio end which would give you a conjugate match if set right and prevent the radio dropping power.which is a similar effect to messing with the lead length.
if you have poor grounding you could wrap both leads between radio and amp and amp and swr meter into a coil to form an rf choke which will reduce common mode current.but the best solution when running power is a good grounded antenna.
bottom line is nothing you do with coax length will change the ANTENNA swr,but it can affect readings elsewhere,especially at the radio end which is undesireable as it could trigger the radios protection circuits and reduce output power.some radios are more forgiving than others in that dept,i've had radios that would tx into very high swr with minimal power reduction and others like the ts50 that shut down tx completely above 2.0:1 swr.
