There is a bunch of information about making a roll-your own leakdown tester, and the orifice size is specified.
http://www.xs11.com/tips/misc/misc3.shtml
One builder says use a 1mm orifice. You just want something to isolate the input air pressure from the residual pressure on the cylinder side of the gauge.
The only thing specific about an aircraft tester is that they always use 80 PSI as the standard, and any A/C leakdown test is expressed in a log book as those numbers, not the percent leakdown, e.g. 76, 75, 73, 72 for a four-cylinder lycoming. Other than that the tester is the same- but airplanes don't use a 14mm spark plug, the typical REM40 plug is 18mm, so be sure to get one that can accomodate an automotive plug.