Complex sentence: Riding halls are not heated because this is healthier for the horses and most arenas have tournament standard dimensions of 20 x 60 metres or of 20 x 40 metres and 25 x 65 metres.
Split into simpler sentences: Riding halls are not heated because this is healthier for the horses. Most arenas have tournament standard dimensions of 20 x 60 metres or of 20 x 40 metres and 25 x 65 metres.