Context :

Aims : An extremely large filament was studied in various layers of the solar atmosphere .
The inferred physical parameters and the morphological aspects are compared with smaller quiescent filaments .

Methods : A giant , quiet-Sun filament was observed with the high-resolution Echelle spectrograph at the Vacuum Tower Telescope at Observatorio del Teide , Tenerife , Spain on 2011 November 15 .
A mosaic of spectra ( 10 maps of 100 \arcsec \times 182 \arcsec ) was recorded simultaneously in the chromospheric absorption lines H \alpha and Na i D _ { 2 } .
Physical parameters of the filament plasma were derived using Cloud Model ( CM ) inversions and line core fits .
The spectra were complemented with full-disk filtergrams ( He i \lambda 10830 Å , H \alpha , and Ca ii K ) of the Chromspheric Telescope ( ChroTel ) and full-disk magnetograms of the Helioseismic and Magnetic Imager ( HMI ) .

Results : The filament had extremely large linear dimensions ( \sim 817 arcsec ) , which corresponds to about 658 Mm along a great circle on the solar surface .
A total amount of 175119 H \alpha contrast profiles were inverted using the CM approach .
The inferred mean line-of-sight ( LOS ) velocity , Doppler width , and source function were similar to previous works of smaller quiescent filaments .
However , the derived optical thickness was larger .
LOS velocity trends inferred from the H \alpha line core fits were in accord , but smaller , than the ones obtained with CM inversions .
Signatures of counter-streaming flows were detected in the filament .
The largest conglomerates of brightenings in the line core of Na i D _ { 2 } coincided well with small-scale magnetic fields as seen by HMI .
Mixed magnetic polarities were detected close to the ends of barbs .
The computation of photospheric horizontal flows based on HMI magnetograms revealed flow kernels with a size of 5–8 Mm and velocities of 0.30–0.45 km s ^ { -1 } at the ends of the filament .

Conclusions : The physical properties of extremely large filaments are similar to their smaller counterparts , except for the optical thickness which in our sample was found to be larger .
We found that a part of the filament , which erupted the day before , is in the process of reestablishing its initial configuration .