Thumbnails are obviously supposed to be kept fairly small - but what is a normal average size for a thumbnail?

When using gesso, do you have to add the calc carb to the gesso, or the Ox Gall to the paint?  Or are those simply to allow deeper penetration of the paint into the gesso?  Can it be used as is?

The plastic nature of straight gesso doesn't allow much penetration of watercolour. Adding Calcium Carbonate helps make it a little more porous. The Ox Gall breaks the surface tension of the watercolour, allowing it to spread and penetrate better. If you cant get Ox Gall, baby soap with a neutral pH can be used. A tiny amount is all you need. It's effect can be seen on a plastic surface - Try painting straight watercolour onto a piece of plastic, then add a little baby soap and see the difference.

As a complete novice, what size paper is reasonable to start with.  I am using A4 size at the moment, but notice that I seem to be a little "tight".  Do I need bigger paper to "loosen" up or will that just come with practice and a surer technique?

A4 is a good size to start. As you become more confident your work will loosen up. It is a good idea to vary the size of the paintings you do. A full sheet might seem daunting at first, but it forces you to take a big bold approach. Working on a smaller size painting seems a much easier task after a couple of full sheets!  

Also, I am using masonite board as a base board to paint on.  Can I use an acrylic sealer on it (ie Clear Floor by Cabots) or does it have to be Gesso?  (I am asking this because I have several litres of Clear Floor in the shed!)

Clear Floor would be fine to seal the masonite. You may have to lightly sand the area the tape adheres to if the surface is very glossy  

I have also read that paper should not be stored in cupboards/drawers that are made from chipboard due to the acid in the glue etc.  If the drawers or whatever are painted with an acrylic paint, do you still have the same problem?

A couple of coats of acrylic paint should seal the acidic glue into the board and make it safe to store paper on. You can buy acid free tissue paper from art suppliers. A sheet of this under your paper is a good idea no matter where you store it  

You mention using a fixative spray on the paintings after using pastels, however, do you generally spray a fixative on straight watercolours?

There is no need to spray straight watercolour with fixative. It is a binder that stops pastel, charcoal, graphite etc. from  smudging. Once a watercolour is thoroughly dry it is quite stable, but does need to be protected behind glass.