What should be the texture size for this default cube

Okay, so first of all this is not the question about that max texture size a device could handle but what i m asking is texture with what size will not get blurred (actually super blurred) on a simple 1 meter cube … Actually i m a programmer and not some designer or texture artist … For testing i made a simple 128128 texture and tried to apply it to a 11 cube (custom cube with all UVs setup to show my texture on all sides) but what i m getting is a super blurred texture but if i m applying a 512*512 texture then i m getting better results … And yes i heard of something called Texture Atlas and did research about so i know what it is but now my problem is that on all unity forums they are saying that i should not make textures bigger then 2048, okay no problem, but if a texture smaller than 512 is getting blurred how can i show 20 textures in a 2048 texture atlas

First of all, the size of the texture needed depends on the distance of the cube to the camera. Very close means a larger texture is needed. Second, every texture can be configured to use MipMaps, which is default. This means ever smaller versions of the texture are created in VRAM so that the graphics card has it easy when rendering far away objects with those smaller textures. These are chosen by a mechanism called LOD (Level of detail). On certain steps along the distance from the camera, these get swapped in and out as needed. It can be configured in Quality Settings under LOD Bias:

The last thing is, that packing an atlas is a good idea, but for Unity it’s only supported for sprites. That means you’d need to create that atlas yourself. A cube then needs appropriate UVs to match the section on the atlas it should show.
And by the way, a texture may even be 4096. They can be even bigger using Sparse Textures, but I have no idea how to use them:

Just as a reference. On a FullHD screen, a texture of 512 should suffice if a cube is covering a quarter of the screen and shows it on each side.