Here's my guess:
- Splitting into fixed size chunks: 2-3x
- Ring signatures with a sane minimum for guaranteeing anonymity: 3x
- Block time: Negligible in the limit as the number of transactions increases
- Bigger individual signatures and addresses: 2x
Overall: Roughly 10x the size of bitcoin. Perhaps 10x, perhaps 30x. With bitcoin at nearly 20GB, the XMR equivalent would be somewhere in the 200-600 GB range.
This is a concern. Once the blockchain is too big to fit on a single commodity hard SSD, for example, the *time and knowledge* needed to participate as a full node in the system increases substantially -- RAID arrays, building your own hardware, etc. The time to download the blockchain may also be a concern for the time to bring a new node into the ecosystem.
This isn't just a concern of disk space, which can be remedied with adding more disks in RAID, people keep forgetting individual nodes have to verify all those scripts in that data in order to individually verify the blocks. It essentially boils down to more centralized mining, and being very prohibitive for smaller development projects to be self sustained and not depend on the third party to trust. I would be very glad to see the projection from the Monero developers how many processor cores would be required for full Monero node to run with amount of transactions getting close to Bitcoins.
Bitcoin runs about 60K transactions per day which is less than one transaction per second. Even allowing for the fact that transaction flow is not constant throughout the day this is not even close to being a serious issue.