As a rule of thumb, if you are looking for a football that will be used by older teens and adults on a full-size pitch, usually 11 v 11, probably outside and possibly in any sort of league, whether it is your local pub league or the Premier League, then you need a size 5 football.
For example, size 5 is used in all FIFA World Cup games and UEFA Champions League matches. If you are purely interested in a kick about between a few friends, then you don’t need to worry about the actual size and weight of the ball, but if you are playing in most league competitions, then the ball should weigh between 410g - 450g, with a circumference of 68cm - 70cm. The actual pressure inside the ball will vary from ball to ball due to its style of construction and the materials from which it is made. The pressure is usually on the ball.
Almost every football manufacturer will produce a full size football. Many of them like adidas and Mitre have official match balls used by everything ranging from the World Cup and UEFA competitions to the Premier League, English Football League and Scottish Premier League.
As with most things you can purchase, you get what you pay for. Footballs range in price from over £100 to under a tenner.
The higher value footballs are made from superior materials and are constructed in such a way as to make them retain their shape, be better for control and passing, have true flight, repel water and, in many instances, comes with a guarantee to cover the stitching or welding.
Size 5 footballs are frequently designed for specific use. The main categories are match balls, training balls, balls for use on certain surfaces like concrete or artificial grass, balls for mainly indoor use or small-sided games, and balls that are purely aimed at people who fancy a kick about.
In other words, the most common ball you’re likely to see and / or need will be a size 5 ball.