Could you tell me the optimum size of the swap partition to SSD?

Don't listen to them=)
1.5 years on SSD with no swap partition.
Swap in an ssd too.
However, if enough RAM, it may be possible. I had the SSD when I had 2GB of RAM — enough head.
In General, you can read my article:
\rhabrahabr.ru/blogs/personal/64682/
Infa somewhat outdated, but generally all right.

Depends on the task. If now it all fits in memory with a large margin is not needed. Does not fit — you need as much as does not fit + a supply.
In General, for servers usually don't do either swap, or make it Vram/2.

> But how to be in this situation, and whether it needs a swap partition in this situation?
\r
a few gigabytes is better to leave just in case, because if you suddenly run out of memory, at the expense of the swap is the probability that he will be able to torknutsya and the system will continue to function.

The problem you have is specific, you need to personally test case, to logs, to know how much maximum RAM is busy. I, for example, 8GB RAM, 40GB SSD and the paging file I put from 64MB to a couple of gigs, never more than 64MB was not, although the RAM is usually responsible for up to 6-7 gigs.

Swap need when there is not enough RAM. Linux (by default, the settings affect the "aggressiveness") of "some other OS" is it smart, not trying to have caches and buffers half the RAM. But there is a negative point — even a program/data that is used very rarely it will keep in memory up to the last instead of trying to unload them for a couple of weeks, and the memory put on the cache. If your ratio of RAM/screw I'd give the swap a gig or two (if not sorry) just in case and in hope that something there to unload, but messages like "can't fork" or "out of memory" it will not save as selection under the swap, say, 16 GB of RAM if 16GB is not enough in 99% of cases it's in the wrong memory consumption because of wrong settings in the software

How can there be a "swap partition to SSD"? Technology for SSD drive don't need swap partition, it is necessary for the system, which this SSD installed. The system can swap, remove. Most web servers run at 40-60% memory usage, the rest is disk cache.