What size do you make a base?

Unless you have to make the dio a certain size to fit your shelf space etc, I'd say the right size is big enough! ;)

Seriously though, the best thing to do is determine what you want to put in it first and make/buy a base to suit the scene. If all you want is a vehicle and a couple of figs to give it a bit of life, you don't need a base much larger than the vehicle. On the other hand, if you're adding buildings, other vehicles etc a much bigger base would be needed.

I agree with what was said above, you really need to work out your composition and then cut a board to size. That being said, you may be able to add extensions if you have miscalculated...Rubens did it with his paintings all the time.

There are two ways to proceed here.

1. You already have a base so you design your scene to fit the base.

2. You have models but no base. here you design a scene that makes sense and clearly, I repeat, CLEARLY, tells a story and you get a base to fit by having placed your elements around beforehand.

A good thing to do in either case is to lay out variations of your scene and take digital photos of it to see which works best and gets your point across. By posting those in progress or planning shots here or on other sites, you may get some good feedback. Ignore those that tell you to do what you want, they're your models/it's your diorama. That's one of my pet peeves and it's as useless at, well, you-know-whats on a bull.

Excellent information! Exactly the guidance I was hoping for, thanks gents!

Ditto what the others said... Compose the scene first in a layout to determine the size of the base needed...  You might have to adjust for space, thats your call, but it's always better to have to much to fill, rather than the other way around...  Once you start landscaping, you can run outta useable space PDQ...   I like to use plastic box picture frames for bases, since they come in a variety of sizes and can be glued together side by side or end to end as needed...  They're cheap, and they generally come in 4 x 6. 5 x 7, 8 x 10, and 11 x 14 inch sizes... Mix & match, then glue and landscape... 

In 1/35 scale, I usually leave around 1 1/2 to 2 inches of space around all the relevant diorama elements. That doesn't include backgroud elements (like building ruins) which can go right up to the edge. In smaller scales, I leave less area.

I try to arrange the elements at an angle, rather than square w/ the base. Also, an oddly shaped base is more visually interesting that a regular square or rectangle.

I usually make a mock-up of all my bases from cardstock before I begin assembling the whole thing. That helps alot.

BTW, if you only buy ONE modeling book, ever, and want to build dios, get a copy of Shep Paine's How to Build Dioramas ...

In the meantime, check out these tip sheets Shep did for Mongram back in the 70s.. This is what me and probably thousands of other diorama builders out there started...

http://sheperdpaine.atspace.com/index.htm

Granted, it's not as in-depth as the book, but it'll get ya started, point ya in the right direction as it were, and spark some ideas too...  The tip sheets are downloadable as pdf files.

All good advice - to echo,,, gather your figures, equipment, etc. Put into a "scene" you are envisioning and then determine the size of the base.

A custom sized piece of pine or oak should not be very expensive.

Best of luck........