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Alexander,
This is a difficult question to answer as we don't even know all the details of the 158 known globular clusters in our own galaxy. The Andromeda Galaxy may have up to 500 globular clusters and M87 in Virgo as many as 10000.
Of the Messier globulars, M71 in Sagitta has a diameter of 26 ly (light years) while M3 has a diameter of 180 ly. A rough average diameter appears to be around 100 light years. 47 Tuc has a diameter of 120 ly and the great Omega Centauri 172 ly.
The small globular M71 has perhaps some 10000 stars while Omega Centauri has several million stars. 47 Tuc and NGC 2808 in Carina have around one million stars. M2 in Aquarius contains around 150000 stars. The average is perhaps in the range 100000 to 500000 stars.
Visually, the brightest globulars (Vmag<10) vary in size from about 5 arcminutes e.g. NGC 6316 in Oph to 36 arcminutes for Omega Centauri at Vmag 3.7.
Regards, Rob
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