Benchwork- are 1x3 cross braces sufficient strength?

Given the stated parameters and access, probably not a problem.......

It can also be a matter of the price of the various sizes and lengths of lumber.

For that size, I think that 1x3 would be sufficient for everything.  Additional strength can be obtained by gluing the 1/2" plywood to the frame.

Here is a suggestion on how to make a module:

If your target is light weight, strength,portability and durability consider using 1/2" Birch A-B plywood .

Have the shop take a 3' X 8' rip, a 3"  X 8' rip and a second 3" X 8' rip that will supply your deck and two side frame members.

Then take the 3' x 8' and cut a manageable deck length like 5' for a 3' X 5' deck.

From the 3' x 3' cutoff have the shop rip five 3' X 3" cross members on 16" centers.

Predrill all cross members prior to assembly for wire management holes.  Always confine all holes to the center third of any material.

Use the precision cut deck panel as an assembly pattern,  lay out the side members and trim to fit, position the cross members and trim to fit.

Cut some glue blocks.  Lay the deck on a perfectly flat surface and shoot it together with clamps and polyurethane adhesive.

Model Railroad Benchwork is a very reasonably priced company.  I would consider asking them if they may supply a pre cut kit.  Or even have them make the straights for you.

I would not think to use dimensional lumber for model RR bench work.  No telling how the wood may warp.  May be OK, may NOT be OK.  'taint worth the gamble IMO.  Remember you are looking for repeatable R&R of the modules.  You need dimensional integrity.

Also, think to request predrilled wire management holes for your curved ends.

Sample photos:

Tight bond for a clamped assembly or polyurethane for a brad shot assembly.  Use corner glue blocks.

Add a bottom skin for an absolute rigid module much like a hollow door blank.

Thank you all for your input and advice!

1x3 would be fine for cross braces, that table would likely be able to take your weight, mine is constructed similarly (ladder rail construction, 1x4 for frame and cross members, braces I think I used 1x3), and I have gone on the table and I am not a lightweight, my legs are 2x2. 

As far as warping goes,there are a couple of pieces of advice I have.1)No matter where you shop, don't get pine, get a hardwood of some sort. It will cost more, but they don't warp as much IME. Kiln dried pine should be fine, but these days honestly the quality just isn't there, especially at a big box store (Lowes and HD to me are neck and neck in terms of race to the bottom with that). I would recommend a lumber yard, I got my lumber there and while it was more expensive than the big box store, it was good quality and has held up. My basement has temp swings, and does have humidity swings even with a dehumidifier, and the word has held up beautifully. I used a 1/2 plywood top as well.