> any recommendations about size and shape of a liner?
We (my wife and I) found that the liner needs to be a bit larger than the SB in practice. The idea of having it smaller to avoid compressing the SB is a good one, but it didn't work out that way in practice.
The liner, being very light silk, does tend to get a bit wrapped around you during the night. If it is smaller than the SB this can be so restrictive that you risk accidentally bursting it - or at least you wake up in total frustration! I find that Sue likes hers about 90+ cm wide - and I make it rectangular as well. These days she uses a quilt instead of an SB, but the need for reasonable width still holds of course.
I moved to making all-encompassing silk pygamas for myself, with hood and feet covers (the latter are a tougher poly-cotton). The first shirt I made was a nice fit for a shirt - but it split down the back when I turned over one night and hunched my shoulders! You need a fair bit of slack for pygamas as well as for a liner.