There is no real annotate feature outside SAS/GRAPH, so you can't really annotate a proc plot. Either use GPLOT or consider the following syntax :
PROC PLOT DATA = ... ;
PLOT Xvar * Yvar = "#" $ labelVar ;
RUN ; QUIT ;
THe # symbol will be used to mark points, and the values of labelVar will be written next to the points.
For "square" graphics with both axis of the same length, try to change the LS and PS option to control the Output page size. You can shrink or expand axes that way (you can do the same with SAS/GRAPH using the VSIZE and HSIZE GOPTIONS). But it's not a very satisfying way to do things, just a workaround.
Maybe you can check also the ODS GRAPHICS templates : maybe there is a "square plot" template that you could use in a Data step (I've seen SUGI papers talking about such things).