As in the title I have a 7d mk2 which I very rarely use now ( also have a 5d mk3 which I use all the time)
I am thinking about selling the 7d2 to get something smaller to carry about when the 5d3 is inconvenient.
Any advice or suggestions very welcome ...
.... It depends on how physically small you want to go and what subjects you like to shoot.
You also say that you already own some Canon L lenses and those of course are high quality and I suggest worth to consider keeping.
You actually have several options but each, not surprisingly, with different costs!
MIRRORLESS EOS M5 OPTION :
A very compact camera body with either its own range of very compact EF-M lenses or mounting EF lenses via an adapter.
Its APSC sensor delivers the same image quality as the 7D-2 and 80D when EF lenses are mounted. I have no experience of IQ with the EF-M lenses.
The M5 Mark II is very strongly rumoured to be announced early next year.
It is a relatively inexpensive option.
Having owned both a 7D-2 and a M5, I personally found the M5 controls too small to be comfortable and unbalanced with any lens bigger than a 100mm but others may feel differently.
MIRRORLESS EOS R OPTION :
In between the sizes of the 5D and M5 and able to mount both EF and EF-S lenses but not EF-M lenses with choice of 3 adapters.
Full-Frame 30Mp image quality on a 5D-4 sensor.
Buying one is buying into a very promising EOS mirrorless future.
A relatively expensive option.
I have replaced my M5 with the EOS R in the last few days and have no regrets.
The 7D-2 is much more of an action camera than the 5D series but you say you rarely use yours currently. Neither the EOS R nor M5 are great action cameras (sports and wildlife) but that's not to say you can't have a go and sometimes succeed.
A.N.OTHER MIRRORLESS SYSTEM :
There are several choices but quite a few reports of them not adapting so well to the Canon lenses, so it might be best to sell everything Canon and start from scratch.
Also a relatively expensive option depending on what you choose.
I have owned 70D, 7D-2, M5, 5D-4, 1DX-2, EOS R and so ask me if you think I can answer any more questions. All my lenses are Canon L.
HTH