The filter standpipe has a 1/16" diameter flow control orfice in it, so even the 1/8" copper tubing that feeds the pressure gauge would flow enough volume. However, 3/16" or 1/4" OD copper/nickel hard line that most local parts stores sell in various lengths would be more durable and less likely to get damaged. It bends easily, with either a basic plier type tubing bender or just barehanded, if you're careful.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!