I have the problem as below,
I'm using TpLink MR3020 and when I install some even LEDE version, I don't have free space enough.
So I would like You to advice me with which very lite version of openwrt I can install on this device ?
It is possible to run 18.06 on the MR3020, but not recommended given that it is a 4/32 device. 17.01 is probably your best bet.
I think that there is a fully functional standard build (complete with LuCI) that will fit on the device, but that fills almost the entire flash space, so you won't be installing many (if any) other packages. If you want to be able to install additional packages, you might consider running an extroot configuration by plugging in a thumb drive into the USB port and pivoting the filesystem to the external storage (this is assuming you don't need the USB port for anything else or that you can use a hub to get extra ports). This thread discusses the image builder recipe to make an image that you can use to bootstrap to the extroot environment, but you will have to leave out LuCI until you are done with the extroot. This means a bit more work on the command line to get your upstream (WAN) connection and wifi up and running so that you can then install LuCI and any additional packages you desire.
Upgrading the flash memory isn't a bad idea in general (provided you have the equipment and skills to replace SMD chips -- not difficult, but not trivial). However, you'll still be stuck with 32MB RAM unless you also upgrade the memory at the same time, and the processor is also a bit old/slow (not to mention the single 2.4GHz 802.11n radio). 32MB is really not particularly great for 18.06+. As such, I would say that 17.01 is the best option for this particular device (with or without the flash/RAM upgrades). I rarely suggest that people run older versions of OpenWrt since there are many security patches and general improvements in the newer versions, but in some cases, hardware limitations dictate the solutions.
If you're looking for a solution that can run 18.06+ comfortably, try the WR902AC (2.4GHz + 5GHz, 8MB, 64MB) or the GLi-net travel devices with 802.11ac, or some of the other more recent travel routers.