I’m looking for a saddle bag to store a spare (26″) tube, CO2 inflator and a couple of cartridges, multitool, tyre leavers, keys / small emergency phone for 1-2 hour local rides where I prefer not to take the Camelbak. I only have one set of bottle cage bosses on my frame so a second storage bottle is out of the question.

On the advice of some online reviews I went for a medium size Topeak Aero Wedge bag but now I’m starting to worry that it might be a little big. Would the small size suffice or would I struggle to get all my bits in?

Nah. Medium sounds about right I think. Better to have a bit more space, than not enough!

I bought a Vaude saddle back in a medium (racelight, or something) and it’s tiny. Can just about get a road tube, tool and some tyre levers in it. Stupid thing, but I didn’t want it coming too far down my seatpost and obscuring one of my lights.
But, really need to replace it with something more sensibly sized….

I think a mate uses a medium topeak one on his mtb and it’s about right.
Just try it when it turns up and send back if it’s too big for the next size down.

Nice one stumpy you’re right, probably best to go slightly large than run out of space.