Can't give you a magic number for size, but some considerations include: a) your ability to stand up from a crouch - do you need to grab / hold on to something to pull yourself up? b) if you use a section that you push up, will you have sufficient leverage to raise it while standing up, and where will you place it without damaging nearby items? A larger section makes more room for standing but it will be heavier to lift and needs more room to set it down. An access hole, rather than a removable section, is simpler, but it's not always aesthetically pleasing.
Is a topside creeper ladder an option? They're versatile, but the long legs need a lot of real estate to maneuver and can be uncomfortable to use.
In a previous iteration of my layout, I had an access hole that was approximately 18" square. As time passed, it became more difficult to pull myself up through the opening - it needed to be larger so that I could use my arms for leverage. The small size also meant that I couldn't bend over to pick something up from the floor.
Make it more accessible than what you need right now - it won't get any easier. Keep in mind that you may need to have tools or supplies in your hands while you stand up.
