Echoing the above welcome -good to see someone new taking the plunge into speaker design / building.
OK, wading into this. I would be careful about jumping straight in with something like the Super Pensil. The slightly smaller standard Pensil 12[P] has what I consider to be the superior balance of properties. It gives up a little on the bottom end, but that is compensated for with a touch of extra gain & slightly tighter control over the driver. YMMV on that front. I designed the pensils, so to an extent they reflect my preferences in terms of alignment etc.
Room size per se isn't much of an issue, in that (short of very large horns) it's difficult to have 'too large' a speaker for a given space. If you have too much of something, it's easy to get rid of it; increasing what doesn't exist in the first place is somewhat harder.
With that said, if you're new to this, and might be moving at some point in the medium term, I'd probably test the waters first by building something smaller rather than going to the immediate expense of a large speaker. See what you think based on experience & then look at moving up in size (and probably cost although that's not a given).
Regarding driver size, beaming, FR etc., I don't entirely agree with all the conclusions, as there are a number of caveats to keep in mind:
-The table linked to regarding beaming is predicated on the assumption of pistonic behaviour and does not automatically apply to wide-band drivers as-is. In some cases (many) it does, but this is not invariable and the frequencies can vary somewhat. It depends on the driver and how it was designed. Those with moderate - deeper cone profiles will tend to become more directional at high frequencies compared to other drivers of the same cone diameter with a flatter profile, e.g. Jordan, MA &
some TB units, which are designed with an eye toward maximising the resonant properties of the main cone at the expense of some nominal response linearity. You can get similar variation with drivers that have additional sub-cones (whizzers) etc., where behaviour varies depending on the mechanical XO frequency and the size & profile of the main & sub-cones. So it's not quite as simple an issue as is often thought, or not when dealing with wideband drivers at any rate.
-Related to the above, I would suggest being a little careful of assuming a ruler-flat on-axis response is necessarily a good guide to what might be called 'practical' performance. It's very tempting to use that as the baseline, and it does have value -I doubt many would argue on that front. However, many wideband drivers have a response trend that is rising at higher frequencies. This is not because their designers are fools who lack the wisdom of us forum-dwellers who obviously think we can do a better job than they can, but because they are trying to compensate for the naturally greater directionality of cone drivers at higher frequencies. Basically, they're putting a lift in the on-axis response to give a wider useable listening axis, which is not necessarily a bad thing, as the majority of people, especially if you don't have a dedicated listening room, do not tend to listen purely on the driver's axis. As a trade-off, it's not a bad one, although it does put them at a disadvantage if you then use the on-axis response as your comparative basis vis-à-vis drivers that are intended to be ruler-flat (or as close as possible) under those conditions.
So much for driver design / conditions.
In terms of size, it's possible to get < 100Hz LF out of 3in drivers, although in outright terms that comes at the price of increased harmonic distortion and limited output / dynamic range, so whether that's worth it depends on your specific requirements & situation. Some of the little 3in units are very nice though; the PS95 and Alpair 5.2 are mentioned; Fostex's FF85wk is another. Quite a few decent TB drivers available too, while the Vifa TC9 and Faital models have developed a following. As far as cabinets go, the TABAQ is probably the best known example of forcing small drivers low & is very popular. Frugel-Horn Lite doesn't go so deep, but is a little more efficient in loading terms as a horn variant. Lots of other options around, so have a look and see if anything appeals.
4in units will get you plenty of other options in terms of drivers & enclosures. The 5in size will get you more LF, at the price of a little less refinement at the top (that's purely a rule of thumb and doesn't automatically hold good -depends on the drivers). As you go up in size, so you tend to get greater sensitivity, more LF output capacity and dynamic range.
If you like the pensils, then if you haven't tried a wideband driver before, I would be inclined to one of the 4in or 5in model sizes initially, which will give you an entry point and some flexibility in terms of adjusting the alignment to your room. Later on, if / when you move, you may want to revise and / or go for something different, be it larger, smaller or whatever. In general, it's good policy to factor the room into your speaker choice or design, since rooms are probably the most variable part of the chain. Either way, welcome & don't hesitate to ask for advice. That's why we're here: to share thoughts etc.
