Is my queston too complicated or too much in it? Or does it make sense? Need help!!!

Consider first determining mirror width based on the vanity for proportion size. Light should be no wider than the mirror. More importantly, having the light output needed for the space if it's the only source. Lots of inspiration photos on this site for you to get a better visual.

Ok, good idea..so if my sink is 36 inch kind of a standalone furniture looking piece, and the top of it is quartz has about a 2 inch overhang, do you think a frameless kind of bevel mirror about 29-30 inches wide would be about right? Judging from the size of the vanity? And I will have a look at some other vanity set ups on this site for more inspiration!!!

Your link above for lighting appears to be a frontier travel link?

I know duh i'm correcting it whoops! Still having trouble copy and pasting the light!

Yay I think I got it!!!!

Very pretty light! Will you have other similar finishes in the room?

I saved this link because i thought it had a lot of helpful resources/advice

https://www.houzz.com/discussions/6421700/vanity-mirrors-height-width-of-electrical-box-placement-for-sconces

Lala girl, thanks I will look at that but my light will be going over my mirror...no space on size and husband wants to use the box where it is for the light...but I will check that out...I cannot get a darn 36 vanity to drop into this space, I am so NON techy!!! There is a shower surround that we put in when took out the tub, it's just white plan to use rain glass or glass no. 62 for the shower doors, the paint in there will be SW Goosemer Veil....thanks for any tips!

What is the wall length from sink top/backsplash to lighting electrical box? That will help determine how tall for mirror, and if you want bottom of light fixture just above the mirror. For example you may want mirror 6 inches above backsplash and 6" below light.

Jackowskib, So the electrical box is 76 1/2 " off the floor...the Vanity is one of the taller 36 inch vanity with the quartz top...the backsplash is 4 inches....so I am figuring that leaves me with 36 1/2 inches usable space up and down between box and backsplash...the light is 29" long , so I thought I need to make sure mirror is at least 29-30 wide...but I'm concerned about how tall the mirrior can be since there will only be 36.5 height to work with? It's a custom cabinet I could make it a tad shorter but don't want it very short like the old one! The top is already done so I can't change the width or depth..

could do a 29 x 29 or 30 x30 square mirror look weird? Or keep it a bit taller than wider? But as you see don't have a lot of height to work with!!!! My husband prefers to keep the box where it is, or othewise that might be an adjustment option!