Hi Nathalie,
Thank you so much for visiting the Disney Parks Moms Panel with your question! We on the panel love helping guests to plan magical vacations.
I like to carry a backpack while I am in the Disney theme parks. I find that it is really helpful to have items like water, snacks, my wallet and cell phone, all in easy reach throughout the day. I carry what I would consider a standard, school-size backpack with me, and I do take it on rides with me. I tend to put it on the floor of the ride and loop my foot through one of the straps for the duration of the ride. I will say that the ride that you mention that is a little tight for me with my backpack is Space Mountain, but I am able to make it work. I have been able to take my backpack with me on every attraction I have experienced at a Walt Disney World theme park.
If you are concerned that your backpack will be too large to go on rides with you, first do check to be sure that it is not larger than the dimensions listed on the
restricted personal items page. If you are still concerned, you can
rent a locker at any of the four theme parks. You could stow your large backpack there and just carry a smaller bag with you.
I hope that these thoughts are helpful for you, Nathalie! Please do let us know if you have any further questions.