You do realize the different between a 151 cm and a 153 cm board is |-----------|
I'm not in the snowboarding industry, but as a regular joe weekend warrior, I've snowboarded a fair bit (15 years of 30-35 day season) - being that I was the same size as you when I started snowboarding, I feel like I know what it's going to be like for you. I have ridden boards ranging in size from 138cm to 178 cm. Trust me... that extra 2 cm is not going to suddenly make the board "way too big" for you. I am 99% sure unless you've been taking lessons the last three years... you are steering the board the wrong way (which takes a lot more energy/effort) - esp if you skidding around on a Burton Blunt 147 cm all this time.
I have taken lessons...I was just thinking that since I was looking at a slightly stiffer board I shouldn't increase the size quite as much. Is my thinking/logic wrong? I don't have that much experience on boards much larger or stiffer so that's an earnest question.