You'll be fine with that size wrist/what you are wearing at the minute. My wrists are around 6.75 and I wear the 300m tunas no probs, fit well and comfortable.
As for the kanji day wheel, I asked the question in one of the new tuna threads, I believe the S23629J1(ie. Uk) won't be kanji. The sbbn jdm version i assume will have a kanji day wheel so maybe look to source one of those if its a deal breaker.