You can scale the army size down to something more manageable. The game is not concerned with figure quantity, only units. We have been playing Hail!Caesar, Black Powder's Ancients sister, using armies based for Impetus. These consist of units of figures on a single base.
Scenarios come from just about anywhere. There are a number of books by various authors, most notably C.S.Grant and Donald Featherstone (apologies to those I have not mentioned) and Battlegames produced a volume called Tabletop Teasers which is a very good source. There are also some printed regularly in the Wargaming press like Miniature Wargames or Battlegames, which give orders of battle. The internet is a good source for OOBs as well. Look up any particular battle and they are usually listed. There is also this rather useful site
My local club often only uses a 4ft X 4ft table with about 8-10 figures per unit and little wooden markers for casualties and disruption. Whilst it will probably horrify some the system works perfectly well. As for scenarios and OOBs then any good history of the period that you want to play should give that. Of course what most people do is build scenarios around the figures and terrain that they already have - it is your game now after all.
Boy, it sure would be nice if we had some grenades, don't you think?
I had originally suggested to my group that we use War of the Ring movement trays as one unit, and I think it would have worked just fine. But, oh, no what about squares and such rubbish. And so we all based them on six 40mm square Victrix bases, 4-6 figures per base so a lot of bloody figures. And guess what. Instead of painting a huge number of small units quickly everybody is bogged down painting 36ish figures per unit. Sigh.
I am building a Sudan British and Mahdist army and doing it my way! My figures for both sides, so WotR trays for a unit. The Fuzzies will get some filled in holes to look more random and horde like, and as well I will stretch out the 160 of the buggers to look like a proper 'orde.
For our Napoleonic games, it's been been huge tables too. I'm only 165cm tall. I can't reach across an 8' table. I'm no Rick Priestley or Alessio Cavatore. Oh no. So my Sudan table will be 4x6 only. No more!
My mate and I decided we would have a game of Black Powder before we went out to the pub.
He pulled up details of the American War of Independance battle of Cowpens on the internet while I got out his 15mm AWI figures and put them into battalions. Cowpens is a small battle with only a handfull of units a side, especially easy in 15mm. Using the stats in the back of the rulebook for AWI ( from the Freemens Farm scenario on page 110 ) the OOB from the internet and a map we found on there too, we set up the historical deployments on a green cloth thrown over his kitchen table.
An hour and a half later, with the Brits smarting from a close defeat, we toddled off to the pub.
Black Powder is great for quick games in any scale and using any amount of figures. It is not a skirmish game ( you do need to work in terms of Brigades, even if these are small ) and was not designed as such, but is great for fighting small battles quickly and getting a realistic result.