Pop-up minium size for comfort access.

See my invisible Table Access Hole (30" by 42") how-to.  The hole is big enough for me on a ladder to change the can light bulb without hunching down over the "city".
    

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.   

The smaller of my 2 popups is 18"x24".  It is a comfortable size, but I would not go smaller.
I can climb out of it onto the layout if needed with a single-step step-stool.

Bob

@hshill posted:

Walking on top getting harder. Mistakes and advice appreciated.

My only popup is 22" x 22", it was big enough for me.

@Susan Deats posted:

See my invisible Table Access Hole (30" by 42") how-to.  The hole is big enough for me on a ladder to change the can light bulb without hunching down over the "city".
    

Susan, lots of great information on that link including other purposes.  Thanks.