Is this the same sign as 'forever'?
They are related but distinct. 'Forever' typically starts with an index finger drawing a circle in the air before moving forward into a 'Y' handshape, while 'infinity' continuously traces a figure-eight entirely with the 'Y' handshape.
How big should the figure-eight be?
Keep it comfortable, roughly the width of your torso. It does not need to be massive, just large enough for the intersecting loops of the figure-eight to be clearly visible to the person you are signing with.
Does it matter which direction I draw the loops?
Not strictly, but most signers naturally start from the center, move outward to their dominant side, loop back through the center, and then to the non-dominant side, creating a smooth, continuous flow.