Very very big. Practicaly huge. In 463 we have the Bulgars separated mainly to two groups (Kutrigurs and Utigurs which have different lifestyles and possibly ethnic composition) and living from the North-Caucasus area through the North-Pontic up to Scythia Minor. The Utigurs haven't yet been conquered by the Turkic Khaganate and the Kutrigurs haven't yet joined the Avar one (and both haven't yet had the respective various influences). While in 700 that stage has long passed - the "Old Great Bulgaria" of Khan Kubrat (reunification of all the Bulgar tribes) has been created, lived through and split between his sons, the Danube Bulgaria of Asparukh had been created (possibly mainly by Utigur tribes, while Volga Bulgaria - possibly mainly by Kutrigur ones) together with a very strong Slavic element and by 700 the new ruler should be Tervel (the later kesar/caesar of the Byzantine empire and "Defender of Europe" against the Arab invasion).
Btw, if you need some specific information on the Bulgars about your mod, just ask.