Although some guidance is available, the DM must concoct rules for a soarwhale gondola
The Arms and Equipment Guide (Mar. 2003) says, "A gondola is commonly slung under a soarwhale to haul passengers," and, "Riding a trained soarwhale requires a gondola or howdah" (89). And, as you've noticed, the howdah (AE 78-9) (200 gp; 100 lbs.) seats only four Medium creatures comfortably, insufficient for most parties (adventuring, birthday, or cocktail).
Unfortunately, that kind of gondola isn't described elsewhere. For example, the Explorer's Handbook (Aug. 2005) discusses aerial lift gondolas suitable for ferrying passengers to and from docked airships (78-9). Also, City of Stormreach (Feb. 2008) mentions in passing gondolas suitable for navigating canals (40) and, similarly, the Fiendish Codex II (Dec. 2006) describes the city of Abriymoch—on the Phlegethos, the fourth layer of Nine Hells—where "scorched and dented gondolas forged of Baatorian green steel" are used by "[b]arbazu ferry pilots [that] pole their way to the client’s destination while burning magma splashes and sprays into the boat" (52). (Like this fine answer's list of alternative airship sources, this only shows the dearth of information available concerning these particular rules.)
However, the Arms and Equipment Guide itself provides statistics for both the Colossal zeppelin (AE 55) (60,000 gp; vehicle) that measures 10 ft. × 40 ft. and the Gargantuan dirigible (AE 55 47) (35,000 gp; vehicle) that measures 10 ft. × 30 ft. I don't know how accurate it is to have the bare-bones maps of these vehicles offer no differentiation between these lighter-than-air vehicles' gasbags and their underslung compartments, but the text makes it seems like entire vehicle is one unit suitable for habitation.1
Thus these airship's sizes seem like a suitable starting point for concocting a house rule about a soarwhale gondola. The soarwhale's light load is significant at over 20 tons, and that's likely capacity sufficient for such a gondola to be constructed from reasonable materials and have capacity enough left over for crew, supplies, an alchemist lab, a petting zoo, a casino, or whatever.
Keep in mind, however, that the Arms and Equipment Guide soarwhale (89) has the entry Face/Reach: 40 ft. by 80 ft./15 ft. and the DM should adjust this for the 3.5 revision to at least 30 ft. × 30 ft. (For comparison, the Colossal monstrous scorpion and spider (Monster Manual 288 and 289, respectively) have Space/Reach: 40 ft./30 ft. while most other Colossal creatures have Space/Reach: 30 ft./20 ft.). So despite the soarwhale's carrying capacity, the DM may restrict a soarwhale's gondola size to one size category less than the soarwhale (much like a mount is limited to accommodating a rider of, at most, one size category littler than the mount).
War Story: Soarwhale fashion show
PCs in a high-level pirate-themed campaign I DMed had grown attached to their ship, but the limitations of ocean travel were slowing them down. So the PCs found a soarwhale, befriended it, and simply slung their entire ship underneath the soarwhale!2
Frustratingly, vehicles aren't often given weights in official material—some of the Arms and Equipment Guide's land vehicles are given weights, yet others aren't, and neither are weights listed for Stormwrack's vessels (95-103) (although its maps are very pretty). Unofficial material isn't much help either—ship weights are absent from Mongoose's Seas of Blood (Dec. 2001), FFG's Seafarer's Handbook (Jan. 2002) and AEG's Swashbuckling Adventures (July 2002), for example.3
Eventually, after some discussion and Web searches, we realized that even relatively small ships like the Atyla are ridiculously heavy. I ruled the soarwhale needed both a suit of +1 easy travel (Magic Item Compendium 10) nonhumanoid Colossal studded leather (3,600 gp; 240 lbs.) and a belt of the wide earth (MIC 204) (10,000 gp; 0 lbs.) (which, as magical clothing, conveniently resizes to accommodate the wearer, and I ruled that since a quadruped can wear a belt so can a soarwhale) to quadruple the soarwhale's medium load to about 180 tons so that it could carry cargo, crew, and the ship itself without slowing it.
Nonetheless, the PCs quickly realized the soarwhale's speed of 20 ft. was still too slow. They rectified this by commissioning a custom magic item like the boots of striding and springing (DMG 250) (5,500 gp; 1 lb.) except that the enhancement bonus was to the wearer's fly speed not land speed and that, instead of boots (which, obviously, a soarwhale can't wear), the item occupied the head magic item slot. The PCs chose to have the magic item look like a top hat.
1 In the interest of full disclosure, my familiarity with airships comes almost exclusively from the Car Wars air vehicles supplement Aeroduel wherein airship gasbags are typically so immense they rarely even fit on the game's map! In that game, the player controlling an airship's usually concerns himself only with the airship's gondola.
3 And folks scoff at the extraordinary ability wild empathy!
2 Of these, I sort of prefer Ships of Blood. While the rules aren't entirely to my taste, it offers an impressive variety of vessels. Although they lack deckplans, the side-view-only illustrations—which I wish I could credit directly to one of the book's nine interior illustrators—are both functional and evocative.
