When you say "bigger", if you mean in actual dimensions I would be VERY careful.
240 and 260's had the battery mounted to the rear of the engine compartment on the right hand side, right below the side access panel. A stock battery comes very very close to touching the underside of the fender, and in order to avoid that, requires a hold down strap / frame. If it were to bounce and make contact with the fender...instant short and possible electrical damage as well as fire. Changing the battery dimension in height would almost guarantee that the posts would touch metal someplace, and don't forget that some of the battery cables are almost too short to begin with.
If by bigger you are referring to it's capacity in cold cranking amps, I don't think anyone will argue, go for it. The battery voltage is still going to be 12 volts, and the current draw from it is going to be defined by your car and it's components and not by what the battery can do, Additional cold cranking amps just means that the battery will have a higher reserve.