There is a luggage area at the front/rear of some cars. Depending where you get on the space may be vacant or full. If you manage to get a seat at the front/rear of the car there is also room directly behind the chair between the seat-back and the wall, I've stuffed luggage in there before. Between those two spots and the overhead, you may be okay if you're travelling heavy.
You can reserve and pay for your shinkansen seat on the computerized touchscreen ticket vending machines, and I think there is a button you can press to switch instructions to English if you don't know enough Japanese. So, no need to deal with an actual person to get your ticket, if you don't want to.
Last time I made a trip through, I had a 28x14x12 (somewhere around that...) duffel bag which fit pretty nicely on the overhead rack. 30x20x15 might be a bit tough to squeeze, those racks are not very wide. The backpack you can probably just fit on the floor under your legs, that's what I did.
So my language embarrassment can be avoided by using a machine??? Does this "machine" recognize in any way that you have a JR Pass???
I just feel bad expecting everyone to speak English wherever I go, due to my seeming inability to manage other languages: glad to know know language won't be much of a problem with booking reserve seats: thats greatWhy would you be embarassed? Most JR ticket offices have English-speaking clerks.
I just feel bad expecting everyone to speak English wherever I go, due to my seeming inability to manage other languages: glad to know know language won't be much of a problem with booking reserve seats: thats great