So a belt can be a wondrous item that continuously applies. This is a pretty standard type of magic item, explicitly addressed in the Estimating Magic Item Gold Piece Values guidelines. The guidelines for that kind of item is:
spell_level × caster_level × 2,000 gp × 2
Where the ×2 multiplier is due to these spells having a minute/level duration.
Both enlarge person and reduce person are 1st-level spells, which means that (with caster level 1st) either effect could cost as little as 4,000 gp by the guidelines.
There are also guidelines for combining effects: any effects after the most expensive one cost 50% more than they otherwise would. For that caster level 1st version, this means we have:
2,000 gp + 2,000 gp × 150% = 7,000 gp
But by default, enlarge person and reduce person cannot be applied at the same time; they cancel one another out. So these effects have to be continuous, but able to be turned on and off.
Command-word items are another kind of wondrous item, which allow you to use a spell effect at will, by using a standard action to activate it. These are usually cheaper than continuous, since you need to re-use that action every so often. Many weapon effects can also be toggled at will, again as a standard action. So turning either effect on or off is likely a standard action. However, this kind of thing is not handled by the guidelines.
Personally, my feeling is that this feature is necessary to make enlarge person and reduce person work together, so it is basically a part of combining those effects. Adding additional cost to make them usable seems pointless.
Extra features, like allowing you to go straight from enlarge to reduce or vice versa, or to toggle more quickly than a standard action, would cost more. Quite a lot more in the latter case; if it was a swift action, that would effectively be quickened rapid enlarge person or quickened rapid reduce person, 6th-level effects. That would cost a staggering 660,000 gp.
So what does it actually cost?
The guidelines are just that, guidelines. Becoming Large or larger is quite valuable (and Small or smaller has its merits, though it’s less significant), so the base 7,000 gp seems very low to me. Permanent size changes are rather rare; for example, you can’t get it as a feat. Races that get Large size with “only” LA +1 are considered very good. It’s very difficult to establish the value of an entire level like that, but I’d say you’re looking at easily three to five times the guidelines’ suggestion (i.e. 20k to 30k).
The 660k of the swift-action version, on the other hand, is too much. It’s good, but it’s nowhere near that good. Probably a tenth of that (66k) would be enough.