What resolution and screen size is generally considered retina?
That crucially depends on the viewing distance in addition to size and resolution. Generally speaking, Apple's "retina" panels used in laptops and external displays (I'm excluding iPad displays for now) have a
pixel density between 216 (Pro Display XDR) and 255 pixels per inch (14" and 16" MacBook Pro).
For an external ≈24" display, that pixel density range would require a resolution of 4480×2520 ("4.5K"), like the 24" iMac's display. You cannot buy a ≈24" display with that resolution; the best you can get is 3840×2160 ("4K"/"UHD"), which is about 185 ppi.
For an external 27" display, that pixel density range would require a resolution of 5120×2880 ("5K"), like the 27" iMac 5K. The problem is that
most "5K" display have since been discontinued, the exception being the yet-to-be-released Studio Display. However, like the LG UltraFine 5K, it requires a Thunderbolt 3 connection for full 5120×2880 resolution which your 2015 MBP does not have.
If, and only if, your MBP has the discrete AMD Radeon GPU, you can run a dual-tile DisplayPort 1.2 5120×2880 display such as the Dell UP2715K or HP Z27q. These require two discrete DisplayPort cables to be connected to your MBP's Thunderbolt 2 ports (which also act as MiniDisplayPort ports), each cable driving one half of the screen at 2560×2880 60 Hz 10bpc ("dual-link SST"). Both have since been discontinued so you'd have to, say, find a second-hand one.
This basically leaves you with the option of getting a "4K" monitor in either 24" (hard to find) or 27" (loads of them).