If you're interested in the geographic scale of a game world, then you need to consider the following points.
- What is the technological level of the world, and how does it affect transport speed?
- What is the terrain on which the settlements may be built?
On land at least, settlements tend to be at most around an easy day's travel from each-other, to allow for people to travel to the next town without having to stress themselves or their beasts of burden. At sea, the equation changes as ships can carry their own supply.
Considering a Pathfinder game, we would expect that, barring the relatively rare exception of magic or exotic creatures, most transport will be muscle-powered, i.e. walking, riding animals (e.g. horses) or riding in animal-drawn conveyances on land, and on water, boats and ships will typically be wind or muscle powered. The following links provide some reference and discussion as to typical and exceptional travel times in this type of environment:
To summarise the relevant data from the above in case of link rot:
On Roads / trails
Level or rolling terrain: 40 miles/day
Hilly terrain: 30 mi/d
Mountainous terrain: 20 mi/d
Off-Road (or unkempt trails etc)
Level/rolling grasslands: 30 mi/d
Hilly grasslands: 25 mi/d
Level/rolling forest/thick scrub: 20 mi/d
Very hilly forest/thick scrub: 15 mi/d
Un-blazed Mountain passes: 10 mi/d
Marshland: 10 mi/d
From this, we can conclude that, depending upon terrain and the quality of the path between settlements, settlements could be anything from 10 to 40 miles apart, though the highest likelihood, assuming that roads have been built, is 20 to 30 miles apart.
You should consider that there won't be cities every 25 miles on average, however. Most of these settlements are likely to be some sort of village which serves as a collection point for agricultural produce and as a way-station for travelers who can't afford to use more exotic means of transport such as magic or flying mounts. Where the logical routes of travel dictate that more commerce will pass through a single area, the settlements become larger. Other factors such as water supply will dictate how large such settlements can become.
In a world with flying mounts, we can expect that some of the settlements that would otherwise be villages could be larger to accommodate riders of the most common type of flying mount - typically within a day's easy flight of each other. However, unless a region was difficult to fly through (extreme mountains, chronically bad weather), the spacing of these would not really depend on terrain. Also, there may well be some smaller settlements in areas difficult to access by land that exist as way stations for flying travelers.
Extrapolating from Pathfinder's Movement and Distance table, some example flying mounts:
- Wyvern (Fly 60'): 6 mi/h, 48 mi/d
- Griffon (Fly 80'): 8 mi/h, 64 mi/d
- Roc (Fly 80'): 8 mi/h, 64 mi/d* (192 mi/d if you allow "sleeping on the wing")
- Sleipnir ("Fly" 80'): 8 mi/h, 64 mi/d
- Hippogriff (Fly 100'): 10 mi/h, 80 mi/d
- Dragon Horse (Fly 120'): 12 mi/h, 96 mi/d
- Pegasus (Fly 120'): 12 mi/h, 96 mi/d
- Kirin (Fly 120'): 12 mi/h, 96 mi/d
- Overland Flight/ broom/cauldron/carpet of Flying / Flying Ointment (variable)
You would need to decide which of the above (or other) was in common use - probably overland Flight spells/items, or hippogriffs, as they are the most easily obtained, though allowance should be made for slower, rarer creatures such as griffons or even wyverns if such creatures are used much. Way-stations for flying creatures/magic users will typically be placed within a day's flight for the slowest in reasonably "common" use.
Finally, you need to consider the size of sovereign nations. These will be dependent upon many factors, and barring a war in progress, a nation's borders are typically tied to some sort of natural boundary such as mountains, rivers, woods, coastlines, etc. The size of a nation depends on its ability to patrol and defend its land, and in turn this will depend on the strength and belligerency of its neighbors - the stronger and more belligerent the neighbors, the more a nation will have to concentrate its defenses to counter them, and will consequently be less able to defend its interior and other borders.
As to the size of your entire world, that is entirely a matter of personal choice. You needn't even define your whole world to begin with, just your preferred area and name a few mysterious far-away places.