Getting ready to start construction on a new layout. I'm planning on 24" aisles on two sides of the layout room. I took a yard stick, held it against the wall and verified that 24" was wide enough for my big fat belly. The benchwork will be 48" high, not including plywood, foam, etc.
If I find the 24" a little tight maybe I'll be motivated to lose some weight.
Mainetrains
'there's something happening here, what it is ain't exactly clear' Modeling the Hard Knox Valley Railroad in HO scale http://photos.hardknoxvalley.com/
The bigest factor for aisle widths is how you are going to operate your layout. If you have fixed operating positions you need to provide space for other operators to get past. I belong to a club with a large walkaround layout. We have aisles down to about 24" but provide people passing sidings where we have fixed operators, where we expect meets or a lot of switching. Look at your layout design and consider how many people would be needed for a maximum operation, where they would need to move about and your fixed positions. This will drive your aisle widths and may result in a track plan change or a reevaluation of how you will operate.