Did some testing...
The chunks loaded around spawn looks to be about a 15x15 box with the actual spawn chunk in the center. Basically equivalent to having a chunk loader in square mode with radius 7.
The player loads 21x21 box with the player at the center.
I might be crazy, but in single player moving spawn to my base seems to give me a decent performance boost. Saves 200+ chunks I don't use being loaded. FPS looks to be 20-30% higher.
Chunks load pretty consistently when they should, but they don't all unload at the same rate.
Using /chunkloaders command I Tinkered around with a chunk loader at my base, moving spawn, and my player. The spawn box seems fairly constant at 15x15. Did not see any chunks loading around its edge once I got the chunk viewer open. The chicken chunk loader had chunks outside the zone pop in/out of being loaded. So even if you set it to the 3x3 box to be loaded typically 5x5 will load with the edge ones going in/out.
With the chunk loader disabled and spawn moved away I noticed my base chunks were little sticky on staying loaded, but they do eventually unload. Closing/opening world they stay unloaded, but going near them they keep themselves loaded for a long time. The base is contained in a 3x3 grid of chunks. Chunks only have MFR, TE , AE, and IC2 (crops only) in them.