The main factor is voltage drop to an inverter and the surge current when switching on.
Start by dividing the Max. wattage of the inverter 12 and then divide again by 0.8 (80% efficiency);
Wattage/( 12*0.8 ) = Amps
this will give you the maximum continuous current that the inverter will draw, however the switch on surge depending on type can often be 50% and higher
If your dealing with a high power inverter i.e. something greater than 500W = 0.5Kw then your going to have to fit the inverter under the bonnet, so make sure you buy one with excellent waterproofing! You won't be able to run long cables as the current will be way to high!
If your a much lower power inverter say 100W or less then voltage drop becomes the deciding factor on the cabling run, you won't want to drop much more than 0.5 volts, you will need to size appropriately see here for a calculator
https://www.12voltplanet.co.uk/cable-si ... ction.html
Finally do you really need an inverter ? There are very few devices these days that can't be run direct from 12 volts, there are also direct inverters from 12 volts to most required DC voltages say for laptops and chargers.
If your thinking of any sort of heating device either space, or water then your on a loser! Mains voltage heating devices or horribly inefficient, they all assume that you have the power of the National Grid behind the socket not a feeble little 120 AmpHour battery which has very limited capacity! for instance first 120AmpHour battery can only be discharged down to
60 AmpHours if you ever wish to use more than a couple of times! So that's 60*12 = 720 WattHours so you won't even get 3/4 of an hour for a 1Kw heater before its flat.