What exactly is eLISA, how will it benefit from LISA Pathfinder and how large will it be?

Question

This answer to What do the gravitational wave findings mean for Evolved Laser Interferometer Space Antenna satellite says:

Given that the LPF is already launched, I doubt that it will be cancelled. I expect quite the contrary, eLISA is a perfect opportunity to do the confirmation of the results found by the LIGO team, and will gain more public interest from the recent news.

and a comment below it says:

It will cover a different frequency range as well. https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_Space_Antenna#/media/File:LISA_and_eLISA_noise_curves.png

and that frequency range corresponds to the one discussed in What produces gravitational waves with “periods between about 100 - 8000 seconds”?

Questions:

1. What exactly is eLISA
2. How will it benefit from LISA Pathfinder
3. How large will eLISA be?

Answer 1

I don't know did anything change since 2017, but here is complete and bulky pdf https://www.researchgate.net/publication/313365623_Laser_Interferometer_Space_Antenna.

What exactly is eLISA

How large will eLISA be?

After several iterations current mission design (LISA, not eLISA anymore) is planned to be:

-Space gravitational detector consisting of 3 identical spacecraft. The crafts will be linked by laser to create triangle space interferometer. The distance between the spacecrafts will be 2.5 million kilometers. LISA will orbit the Sun at approximately the same orbit as Earth, but at distance 50-65 million kilometers from our planet to minimize gravitational perturbations.

Orbits of the spacecrafts can be seen here:

https://upload.wikimedia.org/wikipedia/commons/thumb/a/ad/LISA_motion.gif/300px-LISA_motion.gif

(interferometer size is not to scale)

How will it benefit from LISA Pathfinder

For correct work the interferometer needs undisturbed test masses in free fall regime in every of 3 spacecraft. LISA Pathfinder was crucial for testing this technology.

Another important test is ongoing on Grace-FO spacecraft - will laser interferometer work as planned.