A scale shows how many units on the ground are represented by 1 unit on paper. In maps, scales are expressed in centimetres,
so 1:50,000 = 1cm to 50,000cms.
Any map scale can be easily converted into more meaningful measurements of distance, metres or kilometres:
Insert a decimal point before the last 2 digits of the scale, so 1:15,000 = 1cm to 150 metres, 1:50,000 = 1cm to 500 metres, etc.
Insert a decimal point in front of the last 5 digits, so 1:100,000 = 1cm to 1km, 1:250,000 = 1cm to 2.5kms, 1:1,750,000 = 1cm to 17.5kms, 1:15,000,000 = 1cm to 150kms, etc.
That depends on what you intend to use the map for, but the scale is the first indicator of the balance between the detail shown on the map and the area covered by it.
The smaller the ratio of the scale, the smaller the ground-area covered, so the map should show that area in greater detail; a map at 1:25,000 will cover a quarter of the area covered by the same size map at 1:50,000, but should show it in more detail.
Conversely, the larger the ratio of the scale, the larger the ground-area covered by the map; a map at 1:1,000,000 will cover a considerably larger area than a map at 1:100,000 but show it in less detail.
But please bear in mind that the amount and quality of detail shown on any map depends on the preferences and cartographic skills of its publisher.
The scale of 1:50,000 is generally regarded as the most convenient for walking, offering the advantage of covering an area large enough to plan a variety of different walks, yet presenting that area in sufficiently good detail. But the scale is only one of the factors to be considered in choosing a map for walking; presentation of the terrain by contour lines, highlighting for waymarked or right-of-way routes open to hikers, GPS compatibility, etc, also need to be taken into account. Another consideration is durability; many maps are now waterproof and tear-resistant.
Many long-distance footpaths are now presented on strip maps, showing the trail with the surrounding countryside divided into several panels to cover a long route on one sheet.
Whilst 1:25,000 maps (particularly, in Great Britain, the excellent Ordnance Survey Explorer series) may usually show greater detail, coverage at that scale abroad varies greatly from country to country and even from region to region and is not always available. Additionally, many national survey organizations (equivalents of the British Ordnance Survey) do not distinguished on their mapping between publically accessible hiking trails and restricted access private footpaths, or publish very small size 1:50,000 and/or 1:25,000 survey maps which divide popular recreational areas between several sheets.
The ideal scale for cycling is between 1:100,000 and 1:150,000 but, as with walking maps, the presentation of the terrain (100kms across the Dutch countryside is not quite the same as 100kms in the French Alps!) and of the cycling routes are of paramount importance. Maps with more detailed scales will cover smaller areas and include information not necessarily required for cycling; maps with less detailed scales and covering larger areas may be fine, depending on their cartography and detail.
There are now also an increasing number of maps or map/guides covering long-distance cycling routes, including along the Danube, the Rhine and many other river trails, particularly in central Europe. And as with walking maps, waterproof and tear-resistant maps will better withstand the rigours of frequent refolding.
Here, once again, the choice is dictated by how large an area the map is required to cover. For exploring the nooks and crannies of the French countryside 1:100,000 is ideal, as is a 1:50,000 Ordnance Survey map in Britain, but larger distances require scales with larger scale ratios. To cover the whole of France or Spain on one side a scale of 1:1,000,000 is needed, but Switzerland or Netherlands can be shown at the same scale as Brittany or Andalucía.
One of the larger scales that road maps are available in, 1:250,000 maps – often referred to as Tour or Touring Maps - provide a good overview of a region or smaller country whilst showing a great level of detail. They’re also useful for cyclists in the absence of dedicated cycle mapping.