Getting it right
Building the tunnel
Two rows, arms up, and a gap wide enough that nobody is walking through sparks.
Short answer
Allow about one metre per guest along each side. Keep the two rows at least two metres apart. Guests hold their sparklers up and angled slightly inward, not straight across.
How long will your tunnel be?
Guests split across two sides at roughly one metre of spacing each. Sixty guests lining up gives you two rows of thirty, so about thirty metres of tunnel. A hundred and twenty guests gives you sixty metres, which is longer than many venue drives.
If the space is shorter than your headcount needs, double up to two guests per metre a side. Do not narrow the tunnel to make it fit.
Width matters more than length
Two metres between the rows is the minimum. Closer than that and the sparks from both sides meet over the middle, which looks spectacular in a photograph and feels alarming to walk through in a dress.
Arms up and angled slightly inward gives you the arch without the crossfire.
Length of sparkler drives the pace
A longer sparkler buys time to walk the tunnel, stop, and turn back for the second shot. On a long tunnel that is the difference between finishing lit and finishing in the dark.
Watch what is overhead
Low branches, fabric awnings, market lights and marquee edges. A tunnel is a line of raised flames and the thing to check is not the ground but the ceiling.