If you are approaching from the NW (Boulder) via Hwy. 353 you will eventually come to an intersection where you will have to turn left (east). This intersection at Emigrant Trail is not marked :) Probably best to figure out the rough mileage from Boulder to Emigrant Trail beforehand. When you reach the final stretch, Road 850 to Big Sandy trail head, where you will turn left (north) there will be mega signage.
I have never approached from Hwy. 28, can’t be of help except that my map indicates 25 miles from Hwy. 28 to mega signage.
Yup, last year my smallish car had no problem on Hwys. 353 and 850.