I strongly suggest reading all the way through the various directions to obtain a good understanding of the whole process before you begin. Also, please be aware that this is not how Roman shoes were constructed The goal is to have recognizably Roman shoes that will last a long time, and are reasonably easy to build.
Plan to use a lot of exacto blades, as it is always better and easier to cut with a fresh sharp blade. I use barge cement (available wherever you buy your leather) for all my leather work. I use 4-5 oz hide for the upper, innersole and outersole, and 8-9 for the bottom sole layers. All the stitching is heavy weight "linen" thread, and I punch holes with an awl before stitching. I use an iron last to do the sole pounding and hobnailing. These methods work for me, and but are not hard rules. I've made many different pairs of shoes, and I find things to do differently each time. I think the basic construction method could be applied to any style Roman shoe. Another idea I've seen is to add an extra layer or so in the heel area, this can provide some added comfort to the modern foot, as well as extend the the life of the shoe slightly. However, real Roman shoes show no signs of having any heel elevation, and if it is too thick it can interfere with the clenching function of the hobnails.
My understanding of proper shoe fit is that the width should be a little on the snug side, whereas you always want plenty of room in the toe area. The heel should also be well cupped, because especially with homemade soft leather shoes there is a tendency for the foot to slide off the sole to one side, to the back, or for the sole to roll around the side of the foot.
In the field of Caliga, I suggest examining Titus Neuroleanus Caliga tutorial, which I think has many good suggestions. First and foremost is the authors recognition of the fact that real Caliga, at least in any pictures of real gear I have seen, rarely if ever have any of the very wide straps traditionally used by reenactors. All the extant Caliga consist of a fine web of straps, leaving much of the foot exposed. I will grant that I have not personally examined the real artifacts, and there may be many not published, but, from what is, thin straps seem to be the order of the day. This website will help you make a very attractive pair of Caliga.