When siliconing glass...

[Megalodon]

Is it better to have a small gap in between pieces so as to allow a tiny layer of silicone to go in between?

Or do you want the glass pieces to be as snug as possible?

 

[Pulp Fishin]

Is this for a sump?
You should be fine with a few mm for room. You don't want the fit too tight or you might break the glass tyring to get it in place f this is for a sump.

 

[Megalodon]

It's for external overflow boxes on a tank and baffles for a sump.

 

[drpo21]

they say to leave around 1/8" of room on each side of the glass when making baffles. i just did mine and it worked fine, just apply a 1/4" bead of silicone and make sure the silicone makes it between the glass, worked for me. then use your finger (lick it first) to gently pressed the silicone into a wedge.
for the external overflow box, i would assume to make it snug by applying the silicone and then pressing the glass firm on it, then do the same as if you were doing the baffle.