Brain aneurysms can be very dangerous. Essentially, they are areas of arteries within the brain which for some reason or another are dilated. With the dilation, the wall of artery is not as strong and more prone to rupture.
The larger the artery of course, the more blood which will be spilled with a rupture. The brain tissue does not do well with hemorrhage and major disability to the point of a vegetative state can happen from a ruptured aneurysm.
What often occurs, is that aneurysms over time will become larger and the larger the aneurysm, the greater likelihood of rupture. What is not clear is just how often small aneurysms rupture.
A recent literature review found that for very small aneurysms of 3 mm or less the rate of rupture over one year was essentially 0%. This rate increased to less than 1% for aneurysms of 5 to 7 mm.
The usual treatment for aneurysm likely to rupture is either surgery or inserting a coil within the involved artery. However, if the rupture rate is reasonably low enough, regular follow up studies every 6 to 12 months to document any size increase has been recommended.
The major problem with this approach is that no one really knows for small aneurysms, the exact relationship between the size of the aneurysm and the chance that it will rupture.
Bottom line, is if you have a known aneurysm in your brain, make sure that you are being monitored by a specialist who is an expert at deciding when and if you need surgery or other procedure to treat the aneurysm.