Modern transformers are electronic and are no longer big heavy things, they are also very reliable
They all work with dimmers
Most are small enough to fit up through the downlighter hole in your ceiling making installation very easy
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 down lighters each with a 35 watt spotlight = 210 watts, we would need 2 x 105 watt transformers
When using with LED bulbs you must pretend the LED is the same wattage as the lamp it replaces for example a 5 watt LED replaces a 50 watt conventional spotlight so you assume it is really using 50 watts when selecting your transformer. There is also a specific LED driver available if you prefer
Alternatively we would recommend you have just one transformer per bulb which makes future repairs much simpler!