The Cruc Mawr description suggests four cubits was seen as tall at the time. Using the standard conversion of 1 Roman Cubit to 17.5 inches (44.45cm), a four cubit man would be 5'10'' or 177.8cm, which doesn't seem especially high (indeed, the 2004 mean height for British males aged 16-24 was 177.2cm*). The average height in 9th C Northern Europe was calculated by Steckel* as 173.4cm, however, this was largely based on Scandinavian data. Comparison of British and Scandinavian heights from the 12th C* suggests Britons were 1.8cm smaller, making 171.6cm. This ties in well with an average height of 171.26cm for men from Medieval English cemeteries*. Either way, 177.8cm is hardly significantly different. However, there may have been a considerable difference between the "Dark Age" Welsh and the Anglo-Saxon/Norman skeletons measured. In a classic early paper Beddoe*, though with a somewhat outdated method, gives an estimate of two Romano-British heights as 169.3cm and 154.2cm, and the latter would certainly make 1.77.8cm tall.
Equally, however, the length of a cubit varies depending of the cubit referred to. Although the Roman cubit is often assumed in cubit calculations, cubits varied tremendously (Info). Trade under Charlemagne (d.814 CE) used the Arabian Hashimin Cubit of 64.9cm (25.56'')*, and given the strong relationship between the Franks and, for example, Offa (Info), this may have been used (giving a "giant" height of 259.6cm). The cubit is traditionally said to be based on the length of the forearm from the elbow to the start of the fingers, allowing for easy measurement of rope etc. Regression formula comparing the humerus to height vary by population and need calibrating to a particular group, however, as an indication, a recent Portugese study* gave a relationship of (3.269 x humerus length) + 59.41cm for men (+/- 8.44cm). At a hand minus fingers length of ~9cm this becomes (3.269 x "cubits" length) + 30cm. Setting this as equal to a body height of 4 "cubits" and solving for "cubits" suggests that for someone to be four "cubits" height or under would require a forearm of 41.03cm or greater. As an indication, a 177.8cm (4 Roman Cubits) man measured today had a elbow to finger-start length of 37cm giving a ratio of 4.78 "cubits" to height.