There are several approaches to dealing with small pieces in copperfoiling:
One approach is to have some of the pieces held in place by over-beaded solder without foil on the tiny piece, but it is patchy at best and likely to lose pieces in the long term.
A very good and strong approach is to partially 'bevel' the edges of each piece on both faces. Grind at 45 degrees until the very edge is only 1 mm thick. Then use foil that is 4 mm wide for 3mm thick glass. For 4 mm glass, you will use 5.4 mm foil. Make sure that the foil covers only the bevelled edges and does not extend outside them.
Solder into the 'V' formed by the bevelled edges. Don't over-fill the joints as you don't want solder outside the 'V'. It also is best if the panel is supported underneath the area being soldered by a wet sponge to more quickly cool the solder.
With the solder contained by the 'V', the solder lines will be of constant width throughout the piece. Best to practice this technique on some scraps before you start the main job.
This approach will minimise the amount of light blocked by the foil - important with tiny pieces - while still providing the strength of fully foiled pieces.
If you have to have really small pieces, just foil them as you would any other piece, and burnish it as normal. Then take a very sharp craft knife (Exacto or similar) and trim the foil so that just a little tiny bit of foil is on the front and back of the piece.
No glass approach
Tiny pieces are really tedious to work with. So if the piece is going to be black or really dark, for example a small hummingbird's beak, or a bird’s eye, don't bother with glass but just fill the space with foil and solder.