Answer: Generally speaking, the texture of a mature arborvitae or mature juniper will be the same as the texture you see on a young plant of the same cultivar. The overall size and shape will be determined by the cultivar you select. A named cultivar propagated by cutting should be quite consistent from plant to plant, although there can be some variation (usually within the stated range) based on the growing site. Seedling grown plants may be more variable.
You might try the Missouri Botanical Garden's Kemper Center site
UConn Data Base has some photos, scroll down and to the left on each entry for cultivar information
Ohio Trees has photos and descriptions
Your local nurseries may have reference photos or established mature plantings in display plantings. Your local county extension may have reference materials or be able to direct you to where you might view mature plants in your area -- quite often public institutions such as colleges and also older neighborhoods will have plantings you can take a look at. It's really better to see them in "real life" if possible rather than rely on a photo.
You might check with local landscape architects and also professional nurserymen to see if they have any reference books that would be helpful.
You might try at the (real life) library and ask the reference librarian for help, books by noted author Michael Dirr might be a good start. His "Manual of Woody Landscape Plants" includes detailed descriptions of the different cultivars including mature size and shape.
I hope this helps!
Best of luck!
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