  (Abridged) Disks are observed around pre-main sequence stars, but how and
when they form is still heavily debated. While disks around young stellar
objects have been identified through thermal dust emission, spatially and
spectrally resolved molecular line observations are needed to determine their
nature. We present subarcsecond observations of dust and gas toward four Class
I low-mass young stellar objects in Taurus. The 13CO and C18O J=2-1 transitions
at 220 GHz were observed with the Plateau de Bure Interferometer at a spatial
resolution of ~0.8'' and analyzed using uv-space position velocity diagrams to
determine the nature of their observed velocity radient. Rotationally supported
disks (RSDs) are detected around 3 of the 4 Class I sources studied. The
derived masses identify them as Stage I objects; i.e., their stellar mass is
higher than their envelope and disk masses. The outer radii of the Keplerian
disks toward our sample of Class I sources are <~ 100 AU. The lack of on-source
C18O emission for TMR1 puts an upper limit of 50 AU on its size. Flattened
structures at radii > 100 AU around these sources are dominated by infalling
motion (v propto r^-1). A large-scale envelope model is required to estimate
the basic parameters of the flattened structure from spatially resolved
continuum data. Similarities and differences between the gas and dust disk are
discussed. Combined with literature data, the sizes of the RSDs around Class I
objects are best described with evolutionary models with an initial rotation of
10^-14 Hz and slow sound speeds. Based on the comparison of gas and dust disk
masses, little CO is frozen out within 100 AU in these disks. RSDs with radii
up to 100 AU are present around Class I embedded objects. Larger surveys of
both Class 0 and I objects are needed to determine whether most disks form late
or early in the embedded phase.
