@adah From what I found when I looked this up, it sounds like having a small caliber basilar artery is good news. It is something they measure on brain MRIs in relation to strokes, and if this artery is distended or elongated, it increases the risk of stroke. and also can cause hydrocephalus or "water on the brain". Most of the literature I found talks about cases where the blood pressure was very high over 200 for the top number which is stroke territory for blood pressure. The top number is the pressure during the heart beat contraction, and the lower bottom number is the pressure in-between heart beats or the resting pressure. I'm presuming that the excessively high blood pressure can change this artery and stretch it out, which is related to a stroke that can occur. Here is some of the literature I found that talked about this. Of course, you need a trained medical opinion about what this means for you specifically, and I can't provide that, but I hope this helps with an understanding of what the terminology means.
https://www.ahajournals.org/doi/full/10.1161/01.STR.0000236058.57880.03
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3123011/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3981198/