- number of tx per day
- distribution of the sizes of tx, how much user can affect it
Available on monerochain.info (first one chart, second one you will have to click around blocks, or use the API
- does mixing factor affect it
The mixing factor has a linear multiplier effect on a portion of the transaction size (input signatures).
- is there any reason to compete for space in 1 block as with Bitcoin
Yes if you are in a hurry. The block sizes can expand (in theory almost without limit but that doesn't work too well in a world with actual limits on bandwidth, latency, storage, etc.) so eventually tx with any reasonable fee will get in. However, that is a slow process, so if there are too many transactions for one block some will have to wait and if you want to compete for priority you will want a higher fee (as with Bitcoin).
- is there any way the user could/should be able to affect the fee
Same as Bitcoin, but not in the code yet. That will likely be added. How prominent it is in the UI is not decided. Some bitcoin wallets bury it down in the preferences where few ever change it.
One difference with Bitcoin is that it isn't feasible to do anti-spam with coin-age because age isn't uniquely determined with ring signatures, so there has to be at the very least a bare minimum fee for anti-spam purposes.
- how did the recent 20x increase of fee affect tx
See monerochain.info but my recollection is about 30% reduction.
- what is the realistic upper limit of blockchain size now, in 1, 2, 3, 4, 5, etc years
Who knows? Your guess is as good as anyone's.
- blockchain prunable to what extent, when?
Prunable is a tricky word (as has been discussed a bit on your altcoin thread). Satoshi defined it a certain way to mean removing already-spent outputs (and the transactions that created them) from the block chain. That can't be done directly because anonymity means it can't be determined when an output is spent (only possibly-spent) and it isn't isn't necessarily desirable because transaction outputs are reused to mix with other outputs because possibly-spent outputs don't become "useless" the way spent outputs in Bitcoin do -- they are used for future mixing. However, there are certainly things that can be done to reduce usage by some linear factors like 2-5. There are concepts for doing something like pruning but they are a relatively long term proposition and not fully developed, so hard to say.
Transaction fees are both anti-spam and also resource management.
If you have other questions ask here or on IRC
