I have some friends who are interested in small reels. This fits well with my machining resources, as my lathe and mill are quite small (Sherline). So I am working on a new design that can be made in several sizes.
An initial step in the design is deciding spool size. For this, refer to an earlier post (19 Feb 2012) wherein I developed a formula for spool volume to be allowed for various sizes of “weight forward, floating” lines. The formula is
V = 0.28 * W + 1.42
where V is volume in cubic inches and W is the line weight (e.g., 5 for WF5F). Additional volume for backing is added separately at 1.1 cubic inches per 100 yards.
Now I am not claiming that this is an exact science; there are individual preferences about how full a spool should be, and some variation between line manufacturers and models. The formula is just my guide, and so far I am satisfied with it.

So here is my initial decision on spool sizes.

Small amounts of backing are considered here. I am still waiting for the trout that takes me to the backing, but I do not like to tie the fly line directly to the spool.

I am wading (figuratively) into new territory in considering 0, 1, 2 weight lines and this makes me uncomfortable. To build confidence, I decided to measure the volume of a small line.
Here are two measurement spools; one is 1.70 cubic inches for a WF1F and the other is 2.54 cubic inches for WF4F (zero backing in both cases).

I bought a new WF1F and borrowed the WF4F from one of the reels that I use. The WF1F is an Orvis “Wonderline Generation 3” and the WF4F is of obscure origin.

Here are the spools with line wound on.

The WF1F took up 1.26 cubic inches and the WF4F took 1.61 cubic inches. The spool with 4 weight has the amount of fullness that I prefer. The 1 weight spool seems to me a little too full, but many people would not agree. I was surprised at how large the WF1F line is.

Rather than revise due to this new information, I am going the leave the design table as it is. If I ever want to fish a 1 weight line, I will make a “size 2” reel for it. One of my friends has a source for silk lines, and may want the “size 0” reel for a 1 weight silk line.

Update (same day): Finished with the post, then installed the WF4F and only 3 yards backing on the smaller of my “Fixed Spindle” reels, which has a spool volume of 2.46 cubic inches.

Looks to me like a good fit. Others would use more backing. I originally promoted this as a reel for WF3F.