Your Via is a shared space, and as such we ask everyone to limit any personal items to a single piece of luggage or a reasonably sized bag. Think of it this way - if the passenger who showed up before you had the same sized item and the car was at full capacity, would your item fit as well? If the answer is no, then we can't guarantee we'll be able to take you and your bag on board.
If your instrument is the size of a piece of hand luggage that you might keep under your seat on an airplane (think, the size of a purse or a briefcase, a viola, violin or clarinet) then you may keep it with you in the vehicle provided that it doesn’t impede the driver partner or infringe on the personal space of other passengers. If you have a larger instrument, like a cello or a saxophone or a guitar, then the above luggage policy applies and the driver partner will try to accommodate it on a space available basis.