Given an ROI on the surface made in SUMA, you first convert that to dataset that covers the whole surface with ROI2dataset (changing the pad_to_node option for the number of nodes in your surface minus 1). Choose the surface to apply the region. Then you can use the info_area option as a summary over all the surface or sum them over the nodes yourself with 3dBrickStat.

ROI2dataset -prefix testroi1 -pad_to_node 198811 -input testroi1.niml.roi
SurfMeasures -info_area -func n_area_A -spec std.141.FT_lh.spec -surf_A pial -cmask '-a testroi1.niml.dset -expr step(a)' -out testsm.niml.dset
++ Notice SUMA_LoadSpec_eng (SUMA_Load_Surface_Object.c:4297 @19:44:34):
Label dsets should only be attached to
surfaces with LocalDomainParent = SAME).
LabelDset ././std.141.lh.aparc.a2009s.annot.niml.dset is ignored.
----------------------------------------------------------
-- total area 0 = 815.8
-- from cmask, nodes skipped = 196773

3dBrickStat -sum testsm.niml.dset'[1]'
815.805