Quite large, I think this is a hand-held live box. Constructed of brass with a nice lacquer finish, the eye-piece aperture is 15mm. The inner lens to the sleeve is 35mm. Length of sleeve 28mm, diameter of inner sleeve 39mm, over-all length 46mm. It has a very strong, short focus and there is no real purpose for the outside sleeve only for it to hold a small specimen in place while the user adjusts the eyepiece.
I will be listing some nice vintage microscope parts and accessories... in the next few weeks. I am happy to combine post with other items. Unfortunately I am unable to answer questions until the 21st.