Often looser than you might think if you're thinking like a v-belt accessory drive on a car. My Newmac furnace (also a Delhi blower, probably smaller than yours tho) calls for 1" slack at center (pushing down on the belt, of course.)
Also, you can adjust the speed pulley on the motor if it's adjustable (can't tell, looks like it might be?), and the belt alignment. If you have an adjustable pulley, the manual it may say how many turns out to set it at for your specific furnace.
Not a furnace repairman and I don't play one on TV, just know enough to keep mine going.