For an ocean wreck dive, I carry a DSMB and a reel that is at least 50% longer than the depth I'll be at. So, generally, at least a 200' reel. I use a Manta CR-2, which I really like and carries 250' of line. I like the soft handle much better than the hard handle on a normal side handle reel.
On the wrecks I've been to, if I have to use a DSMB at all it would be because I have gotten away from the wreck or can't find the up line. Either way, I'm doing a free ascent. And if I'm doing a free ascent, I'm sending an SMB up immediately, so it will hit the surface as close to the boat as possible. If there's a serious current and I'm at 120' depth, the line needs to be longer than 120' for the bag to be able to stay on the surface and not get pulled under by the current.
The closer the bag is to the boat when it hits the surface, the more likely they'll see it. If surface conditions are rough and I were only using a 100' spool, then the bag might not get to the surface until I'm at 60' or so of depth. If I started at 120', that's at least 2 minutes after I've left the bottom and am getting pushed along by the current. Depending on conditions, I feel like that 2 minutes worth of distance I've drifted might be the difference between the boat seeing my bag or not.
I was out on a boat off NC last summer and a guy hit the surface about 40 yards from the boat, with his safety sausage inflated. The current still carried him past the end of the tag line before he could swim to it. Surface swell was 3 - 4 feet and it wasn't long before drifted far enough that it was hard to see him and you could only see him by watching and waiting for a swell to lift him up. The effect of a good current on you during a free ascent should not be taken lightly.
I only bother to carry a spool if it's a drift dive in shallow-ish water.
My SMB has a mesh pocket at the base. I keep my whistle in there.
I have never bothered to carry a spare mask on a recreational dive - except when it was a dive in very low visibility, where I was concerned that my buddy might accidentally kick my mask off without me being able to see it coming. When I do carry a spare mask, so far I have just been wearing my DGX pocket shorts and putting the mask in the left pocket. I just got some pockets to glue on my dry suit. If that works out well, I'll probably get some more to glue on one of my wet suits.
At $50, I have found the pocket shorts to be an excellent investment.
DGX Neoprene Pocket Shorts | Dive Gear Express®
When I wear them, I put my pre-rigged reel and SMB in the right pocket. When I don't wear them, I clip the reel/SMB to my right waist D-ring.