Some of the worldgen parameters determine whether or not to reject a world. Turn all of those off and you won't get any rejects.
In theory, anyway. I still got one reject with this one. Not sure why, and I didn't have the worldgen log turned on. Ah well.
Here, use mine and tweak as necessary. The Initial/Final (biome) Square/Region Counts, about halfway down the list, and the Minimum Number of Mid/Low/High (parameter) Squares, at the bottom of the list, are most of the ones that cause rejects (In the text file, they're [REGION_COUNTS:(biome):0:0:0] and the last five). Even with normal sizes and parameters, I just leave them all off to save time, the worlds don't look particularily different than the ones with them on anyway.
[WORLD_GEN]
[TITLE:RINGWORLD]
[DIM:257:17]
[EMBARK_POINTS:1274]
[END_YEAR:1050]
[BEAST_END_YEAR:200:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:800:52]
[RAINFALL:0:100:400:26]
[TEMPERATURE:25:75:400:26]
[DRAINAGE:0:100:400:26]
[VOLCANISM:0:100:400:26]
[SAVAGERY:0:100:400:26]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[TITAN_NUMBER:3]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:22]
[GOOD_SQ_COUNTS:6:66:132]
[EVIL_SQ_COUNTS:6:66:132]
[PEAK_NUMBER_MIN:3]
[PARTIAL_OCEAN_EDGE_MIN:1]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:1]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:26:26]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:6]
[NON_MOUNTAIN_CAVE_MIN:13]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:20]
[TOTAL_CIV_POPULATION:15000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
I gave it a spin. Unfortunately, 17 is minimum smallest, and 257 is maximum largest when it comes to worldgen size. For those interested, it actually genned VERY quickly. Ringworld style might be a good way to get alot of space without taking forever to gen.
Here's what it looks like as of 1050.
(http://img713.imageshack.us/img713/5302/historicalmapregion2105.png)