Max Texture Resolution & Memory Footprints

[Sitrec]

According to the wiki 4096x4096 is the highest supported but 8192x8192 works if you change the source to allow 14 mip maps.

Engine Texture Resolution Limit

Unreal Engine 4 defaults to limiting the maximum number of texture mips to 13, which effectively limits the largest rendered texture to 4096 (1x1 to 4096x4096 is 13 mips). This has the side-effect that imported 8192 textures will only render up to mip1 of 4096. The constant MAX_TEXTURE_MIP_COUNT which defaults to 13 in the engine source files can be modified to a value of 14 to support 8192 texture rendering. This constant is defined in the following source files (as of QAMar09, be sure to verify on other QA versions).

Textures