First a note about creating sets you intend to combine: make sure properties like length and variable type are consistent when the data sets are created.
You can avoid the message and possibility of truncation but providing a Length statements for such variables before the SET statement: Below assumes that the variable Segmento is character and that the longest length defined for any of the datasets is less than or equal to 100. Use your preferred value if 100 is too long.
data all_res_smd;
length Segmento $ 100.;
set res_smd_:;
run;
A side affect of this is that Segmento will appear as the first column in the data set as it is the first variable used in the data step.