I think you will find that both slavic and western non-slavic linguistic references to (Ruthenian, Polish, Great Russ) sources from the period refer to knaz, knyaz, kniez, knez and so forth as a contextual equivalent of "lord". A Knaz might rule a large area, or a single village. Veliky was oft used in much the same way as we might use the word "high" or in terms of the "dux" or "duc" and refer to either local or not-so-local premiership in terms of mutual defence where there might be the interest of more than one local thug and his hovel at risk. (Perhaps there was an Arthur at Camelot, and if so, he was high king, but certainly rule England he did not)
Very occassionally, briefly and incidentally might there be some greater importance attached to the person wearing such a moniker, but the practical or long lasting effects of such an association can best be examined from the paucity of historical record in the surrounding kingdoms and their interests. Had there been a considerable and consistent presence in the area the church, Poland, Lithuania, even Byzantium, the Mongols, Turkomans, and god knows who else, would have had very very different diplomatic and military patterns. Their records would reflect otherwise, and historical boundaries would not have acted as they did in the period in question, had there been a viable political/military presence for more than a decade or so at a time not totally dominated by internal struggle for cohesion.
Though not without their own subjective biases, papal and other documentation by church officials can paint a fairly vivid picture of where things lay and how they got there...(This is mostly because these folks were the only people-especially the farther east you went- who could write).
Although not all of these writings exist within the confines of Papal control, many being in either private writings or state records of the political organizations of the era, a good number of these can be seen (or their facsimiles) in papal records and treatises on Church policy. Some of them can even be seen electronically through the Encylopediae Caltholica (online) and its very warrenish and (excuse the expression) "Byzantine" structure and links. The more intriguing of these usually will lead you to the Vatican's site, and though they will tease you there, you will have to access them through a library or get permission to visit. If you are an Academic Bigwig, perhaps they would send you a copy, but probably not.
Two other sources in the English language to look at are a number of works published and or circulated as academic working papers by Richard Davies, another is work done by Michael Petrovich. You can see some of his material online as well. I believe (if you have the patience to use Real Player, you can even see this Agustian and hear a few of his lectures here:
http://palimpsest.lss.wisc.edu/~creeca/
