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We describe a robust Bayesian statistical method for determining Lyman alphaforest cloud sizes in spherical and in thin disk geometries, using absorptionin adjacent sightlines toward closely separated QSO pairs and groups, applythis method to the available data, and discuss implications of our results formodels of Ly alpha clouds. Under the assumption of a population of uniform-size and unclustered clouds, the data from Q1343+2640A/B give a 99% confidencelower and upper bounds 61<R<533 kpc/h on the radius of spherical clouds at zabout 1.8, with a median value of 149 kpc/h [$(\Omega_0, \Lambda_0) =(1,0)$].The baryonic mass of such large clouds is comparable to that of dwarf irregulargalaxies. Their cosmic overdensity is close to the turn-around density butgenerally below the virialization density, suggesting a population of gravi-tationally bound but unvirialized protogalactic objects at z about 2. Theircomoving volume density is similar to that of the faint blue galaxies (FBGs) atthe limiting magnitude B of 26-27. The dynamical collapsing timescale of over-densities like these clouds is also comparable with the cosmic time differencebetween z of 2 to 1. Both populations of objects show similar weak clusteringin space. All this evidence suggests a possible identification of Ly alphaclouds as the collapsing progenitors of the FBGs at z about 1. We alsoinvestigate the other QSO pairs: Q0307-1931/1932, Q0107-0232/0235, and thetriplet of Q1623+268. Imposing an uniform W_0 > 0.4 A threshold on alllinelists, we find a trend of larger inferred cloud radius with larger properseparation of QSO pairs, significant at the 3.4 sigma level. This indicatesthat the idealization of unclustered, uniform-sized clouds does not accuratelydescribe the Ly alpha cloud population.

The Size and Nature of Lyman- alpha Forest Clouds Probed by QSO Pairs and Groups