The images below show 625 square metres (with a tennis court backdrop) to give some idea of scale. 25m x 25m is 625 square metres.
Each grid is 5m by 5m.
0.5 people per square metre (312 people in 625 square metres)
1 person per square metre (625 people in 625 square metres)
1.5 people per square metre (937 people in 625 square metres)
2 people per square metre (1,250 people in 625 square metres)
2.5 people per square metre (1,562 people in 625 square metres)
3 people per square metre (1,875 people in 625 square metres)
3.5 people per square metre (2,187 people in 625 square metres)
4 people per square metre (2,500 people in 625 square metres)
4.5 people per square metre (2,812 people in 625 square metres)
5 people per square metre (3,125 people in 625 square metres) - Maximum
5.5 people per square metre (3,437 people in 625 square metres) - HIGH RISK
6 people per square metre (3,750 people in 625 square metres) - HIGH RISK
From the links below you can view a 10m grid, against a tennis court backdrop, to assist in visualising crowd densities and crowd sizes. I'm going to produce a whole range of crowds against backdrops to act as both a training aid and to provide reference visuals for assessing both density and crowd numbers.
Tennis court - reference crowd 100 square metres 1
Tennis court - reference crowd 100 square metres 2
Tennis court - reference crowd 100 square metres 3
Tennis court - reference crowd 100 square metres 4
Tennis court - reference crowd 100 square metres 5
Tennis court - reference crowd 100 square metres 6
Tennis court - reference crowd 625 square metres 1
Tennis court - reference crowd 625 square metres 2
Tennis court - reference crowd 625 square metres 4
Tennis court - reference crowd 625 square metres 5