Well I will start one last try to convince you of the great benefits of a single caburetor design. But to add further information, I will try answer your question first
The carb size is not linked directly to total engine or cylinder displacement. The size of the carb is determined by the amount of air-fuel-mixture needed. (Thus, four-strokes need smaller carbs than two-strokes of equivalent size and rpm).
Small carbs are good for small air requirements, but they are restrictive to higher amounts of air. With small amounts of air, a big carb will not be able to vaporize fuel into the air properly, so you can't archieve smooth running at low engine rpm, when air requirements a very small..
So a small, high-revving engine with bad idle requires a similar carb as a big, low-revving engine does for sufficient performance.
→ Long stroke engines with camshaft timings focused on lower revs with a high amount of low end torque require smaller carbs than short stroke engines with wild cams do.
I hope you now understand why you can't pick the right caburetor by simply taking one from an engine with similar displacement. You have to choose one that fits your engine's individual charateristics. Small changes on the carb setup cause huge differences.
So I strongly recommend one single carb for all eight cylinders in the beginning.
Only one carb to exchange,
only one to adjust.
And the best point: No need to syncronize.
If you still want to go for eight carbs in the beginning, a good idea is to build a singlecylinder engine with 1/8 of displacement first so you can test different carb designs and sizes on that single cylinder to archive smooth run at idle and desired maximum poweroutput (could be tested easily by using a propeller and mesuring max rpm). Then use eight of these carbs on the eight cylinder engine.
Ok, one last try: A single carb for all eight cylinders will produce a deeper and more vibrant resonance of the induction sound because of the combination of bigger induction manifold volume and longer runners.
