As the respected Syed has showned, several sources do witness to the height of Abil Fadhl al-Abbs (pbuh). I would like to add a small note here: Sometimes, we tend to use hyperbolic expressions in our day-to-day language. This is common in many languages. For example, in English you could say "tall as a giraffe", or "as hungry as a wolf", or "I could eat a horse".
Being consumers and contributors to the common parlance of the day, we realize that these are hyperbolic expressions; idioms used to express that something is out of the ordinary. Indeed, with time, even hyperbolic expressions tend to become more common, and so we find that expressions like "awesome" and "fantastic" are used in everyday language, whereas their usage previous was limited to extra-ordinary phenomena.
Given above, I do believe that it would not be far-fetched to understand the statements about the height of Abil Fadhl al-Abbas (pbuh) as hyperbole. That is to say, he (pbuh) was very tall; taller than other people of his time. A way for historians to accentuate this, using the parlance of their day, was to describe his height very tall even when sitting on a horse.
And God knows best
In need of your prayers