John,
The only method of which I am aware (and thus for what my knowledge is worth) that really works for reliably and consistently cutting small planks to a precise and even width is to use a small, table top rip-saw of decent quality, such as the (eminently drool-worthy) one from Byrnes Model Machines (**LINK**), the Proxxon FET (**LINK**) or some other such one.
However, given the ouch! factor of the price tags on these machines, unless you are planning to rip most of your own modelling timber in the future or can otherwise defend the aquisition of one, this is not likely to be a realistic solution to the problem at hand.
The really easy way out would be for you to look up the "timber" sections on e.g. the Cornwall Model Boats (**LINK**) or Model Dockyard (**LINK**) websites, and order the quantity needed of planking already cut to your desired width.
If you neverthless want to have a go at cutting your doll's house floorboards to width without investing in an expensive rip saw, you could try setting up a small jig on a piece of cutting board that would allow you to push the timber to cut up against an edge of some kind that was of the same thickness as the material to be cut, so that the straight edge could be rested perfectly flat on top of the two together. The jig will also need a further, raised edge at the back, perfectly parallell with the edge against which you just put the timber, and at just the distance that when you lay the straight edge against the back edge, and the timber against the front edge of the jig, the amount of timber showing under the straight edge is the desired width (plus maybe a few tenths of a mm for final sanding purposes).
If you then use a really sharp and thin hobby knife or scalpel, and use multiple passes to slowly and carefully cut (not split, as a split is likely to veer off true with any grain in the timber), you might be able to produce deck planks that are sufficiently straight and even to suit your purposes. You might also want to figure out a way to fix your jig to your cutting mat or other work top surface, if in no other way then at least by taping it down with some masking tape.
For small quantities and with a reasonable amount of care and patience, this could work, although the quality of the result would also depend of how finely (or coarsely) grained a timber it is you want to cut. Personally, and until I've managed to save up for one of those rip saws (oh, yes — they're very much on my wanted list!), I would probabaly take the easy way out and buy ready-cut timber to fit the dimensions I was looking for. The floorboards will surely come in handy for some other project some other time …
Mattias