The Horse that the viking age used were very small, they are still around in Iceland today. It would not be too difficult, to not be able to ride such a horse if a person was tall, In Iceland even today riding those type horses, shows that many men have to lean backwards to have there feet clear.
Rollo,s nickname makes common sense, as he was believed to be tall. It was possibly the same sized Horse also used in Anglo Saxon England. The Normans no doubt began to use larger Arabian type horses, acquired through there experience's on their travels, and adapted their fighting skills accordingly. It, was possibly this 'difference' that resulted in the Norman victory at Hastings, by his descendant William.
Regarding the Update, Dna etc bones. I recall they had some problems, and were unable to prove a connection to Rollo.