Research by Danish FA of 100 children's matches recommends 8v8 for 10-13 year olds

This research is based on a total of 100 children’s and youth matches, which were recorded, analysed, and reviewed.In total, around 600 U8 to U14 players participated in the study (including both boys and girls)

'We noted that although there were many ball possession per player in the 9v9 game  most passes occurred across the field - a type of passes which cannot be considered as particularly challenging for the players.'

English Version of Report Here  Download Spilleformer_Banestørrelser_Supplerende_undersøgelse_ENGLISH_2013_HV

Danish FA website including videos here http://www.dbu.dk/spilleformer