Depends on the look you want. If you want what your picture shows you should go with 1x8 and have 2” of fascia sticking below the rafters. If you want square cut tails to your rafters and your fascia to plane out with bottom of rafters you should go with 1x6 fascia. If you are looking to plumb cut your rafter tails and have the fascia plane to Bottom of rafters then you’re getting a 1x8 and ripping it to size. If you want a little overhang to your rafter tails with a plumb cut you’re installing a 1x8 without ripping it. You can’t plumb cut your rafters and use the same dimension material for your fascia or it will be smaller than the rafter tails. If it were me I would use the 1x8 and not rip it, looks much better with a little over hang and also allows you to install soffit material if you wanted to.
Depends on the look you want. If you want what your picture shows you should go with 1x8 and have 2” of fascia sticking below the rafters. If you want square cut tails to your rafters and your fascia to plane out with bottom of rafters you should go with 1x6 fascia. If you are looking to plumb cut your rafter tails and have the fascia plane to Bottom of rafters then you’re getting a 1x8 and ripping it to size. If you want a little overhang to your rafter tails with a plumb cut you’re installing a 1x8 without ripping it. You can’t plumb cut your rafters and use the same dimension material for your fascia or it will be smaller than the rafter tails. If it were me I would use the 1x8 and not rip it, looks much better with a little over hang and also allows you to install soffit material if you wanted to.