Garden railways were historically the most popular, largely because the smaller scales didn't really exist, and power by clockwork or live steam, as was common, was more suited to longer runs. Modern electrically driven models are, however, equally suited to the great outdoors and there are many people building extensive, and not so extensive, garden railways. The most popular scale for outdoors is probably O scale.
Scales smaller than OO/HO are not really suited to the outdoors, although some do exist, but the relative size of garden debris and the visual impact of small trains amongst big plants is not ideal. O gauge is the most popular gauge in the Uk for garden railways it being big enough to cope with the real world conditions but small enough to feasibly allow some form of layout in a garden shed or garage and you are likely to want to have an indoor part of your layout. This can be as simple as giving you a place to set up and store your stock but often forms a layout in its own right, a terminus station perhaps, giving you the best of both worlds.
Have a look at Padfracombe, James Aitken's garden layout to get a flavour of what is possible.
There are bigger gauges used in the garden, often including the use of live steam locomotives, but these tend to be a major investment and will require some investigation on your part. There are specialist groups for these types of models and most clubs would be able to help point you in the right direction.