I would pay attention to the product specs.

How much is insertable. Subracting that from the overall length gives you a good idea of where the bullet (or the bottom of the bullet, at least) will be (because the point where it stops is where the bullet is). IE an 11 inch vibe with 5 insertable inches will have the clit attachment somewhere about 5 inches down the shaft. A shaft too long will position the clit attachment further from your clit. I've had some which wind up having to be inserted to the point of discomfort to get my clit attention.

I think length of the clit stimulator is often a big issue. Many of them are too short. I know the space between my vagina and clit is at least 2 inches (maybe more, I've never exactly taken a ruler out). Floppy ears and paws and tentacles and other 'extenders' can help with this. I also find that larger bullets as clitoral stimulators work better for me because of the distance, which I mentioned. Unfortunately, I don't think there's a easy way to measure size/length of most clit stimulator; at least, it's not listed anywhere.