Modern transformers are electronic and are no longer big heavy things, they are also very reliable.

They all work with dimmers.

Most are small enought to fit up through a downlighter fitting making installation very easy. the 60 watts are matchbox size.

To work out what size you need count how many bulbs you have or want, choose a wattage and multiply that by the number of bulbs, this will give you the size of transformer you need.

example 6 downligters each with 35watt spotlight = 210 watts, we would need either 2 x105 va transformer each running three fittings or one 210 transformer if you prefer.