Christopher Li wrote:
I get slightly more hits on Google for "big" than for "okina", which is usually a fair indicator of usage (Aiki News in Japanese uses "big"). Also, Sokaku Takeda used the kanji "dai", which implies a kind of precedent for that usage. OTOH, Gozo Shioda used "old" ("ro") when writing about Ueshiba in "Aikido Jinsei".
An added note to my own post - I checked to make sure, and Kisshomaru Ueshiba seems to have used "big" ("dai"), which is fairly convincing to me.