Dear all, in order to contribute something useful for a change I have 
compiled a table of spanner (and socket) sizes for the more common metric, 
A/F, BSF, Whitworth and BA nuts and bolts.

Please remember:
Metric and A/F bolt and nut sizes are hex sizes, measured between two 
opposite flats. BSF an Whitworth are bolt diameters, measured over threads. 
God only knows what BA sizes are. The measurements are not the hex sizes, 
but the spanner sizes; hexes would be about 0,1 mm smaller (depending on the 
state of my tools). Please read all commas as periods, if that makes a 
difference to you.

Size      Standard  Spanner distance in mm

6         BA         4,9
5         ISO        5,1
1/4       A/F        5,5
6         ISO        6,1
4         BA         6,3
7         ISO        7,1
5/16      A/F        8,0
8         ISO        8,1
2         BA         8,3
1/8       WW         8,7
3/16      BSF        8,7
9         ISO        9,1
3/8       A/F        9,6
10        ISO       10,1
0         BA        10,5
11        ISO       11,1
7/16      A/F       11,2
3/16      WW        11,4
1/4       BSF       11,4
12        ISO       12,1
1/2       A/F       12,7
13        ISO       13,1
1/4       WW        13,5
5/16      BSF       13,5
14        ISO       14,1
9/16      A/F       14,3
15        ISO       15,1
5/16      WW        15,3
3/8       BSF       15,3
5/8       A/F       16,0
16        ISO       16,1
17        ISO       17,1
11/16     A/F       17,5
3/8       WW        18,1
7/16      BSF       18,1
18        ISO       18,1
19        ISO       19,1
3/4       A/F       19,2
20        ISO       20,1
13/16     A/F       20,9
7/16      WW        21,0
1/2       BSF       21,0
21        ISO       21,1
22        ISO       22,1
7/8       A/F       22,4
23        ISO       23,1
1/2       WW        23,5
9/16      BSF       23,5
15/16     A/F       24,0
24        ISO       24,1
1         A/F       25,6
9/16      WW        25,8
5/8       BSF       25,8
5/8       WW        28,0

This may help if you can't reach the correct spanner while you are wedged 
under the car, so memorize it, please. However, the right tool is the best tool!