The verb in Isa 42:5 translated "spread forth" is רָקַע whose meanings are given in the appendix below. This verb occurs 11 times in the OT as follows:
- Ex 39:3 - They hammered out thin sheets of gold and cut threads from them to interweave with the blue, purple, and scarlet yarn, and fine linen—the work of a skilled craftsman.
- Num 16:39 - So Eleazar the priest took the bronze censers brought by those who had been burned up, and he had them hammered out to overlay the altar,
- 2 Sam 22:43 - I ground them as the dust of the earth; I crushed and trampled them like mud in the streets.
- Job 37:18 - can you, like Him, spread out the skies to reflect the heat like a mirror of bronze?
- Ps 136:6 - He spread out the earth upon the waters. His loving devotion endures forever.
- Isa 40:19 - To an idol that a craftsman casts and a metalworker overlays with gold and fits with silver chains?
- Isa 42:5 - This is what God the LORD says—He who created the heavens and stretched them out, who spread out the land [אֶרֶץ] and its offspring, who gives breath to the people on it and life to those who walk in it. (Compare Gen 1:9, 10 where אֶרֶץ means the arable land.)
- Isa 44:24 - Thus says the LORD, your Redeemer who formed you from the womb: “I am the LORD, who has made all things, who alone stretched out the heavens, who by Myself spread out the land [אֶרֶץ]. (Compare Gen 1:9, 10 where אֶרֶץ means the arable land.)
- Jer 10:9 - Hammered silver is brought from Tarshish, and gold from Uphaz—the work of a craftsman from the hands of a goldsmith. Their clothes are blue and purple, all fashioned by skilled workers.
- Eze 6:11 - This is what the Lord GOD says: Clap your hands, stomp your feet, and cry out “Alas!” because of all the wicked abominations of the house of Israel, who will fall by sword and famine and plague.
- Eze 25:6 - For this is what the Lord GOD says: ‘Because you clapped your hands and stomped your feet and rejoiced over the land of Israel with a heart full of contempt,
Notice that this verb does not imply anything two dimensional but is often used of thing three dimensional like hammering gold over an idol. Its use of "stamping feet" implies a meaning that includes hammering or squeezing out over a surface of whatever shape.
Thus, when God spread out the the land [אֶרֶץ], it only implies the power of God being manifest by hammering it out over the surface of the earth (it is only a metaphor). Compare Gen 1:9, 10 where אֶרֶץ means the arable land.
Thus, I would beg to differ with Barnes - the verb implies a spreading out over a surface of any shape, not necessarily flat.
APPENDIX - BDB Entry for רָקַע = "spread forth".
[רָקַע] verb beat, stamp, beat out, spread out (Late Hebrew id.; Hiphil, Aramaic רְקַע Aphel, both spread out, and derivatives; Syriac press down (Luke 6:38 ᵑ6), spread out, also consolidate; Arabic patch, put on a patch, repair, compare Christian-Palestinian Aramaic patch, swaddling-bands, SchwIdioticon 90; Phoenician מרקע platter, or bowl, of gold CISi. 90, 1); —
Qal Imperfect singular suffix אֶרְקָעֵם 2 Samuel 22:43 I will stamp them down (probably gloss to אֲדִקֵּם preceding, of. BaePs 18:43); Imperative רְקַע בְּדַגְלְךָ Ezekiel 6:11 beat (stamp) with thy foot, in token of contemptuous pleasure, compare Da ("" הַכֵּה בְכַמְּךָ); so Infinitive construct suffix רַקְעֲךָ בְּרָ֑גֶל Ezekiel 25:6 ("" מַחְאֲךָ יָד); Participle active as substantive construct (Ges§ 65d) רֹקַע הָאָרֶץ (i.e. ׳י) he that (beateth out) spreadeth out the earth Isaiah 42:5, רֹקַע ׳הַא Isaiah 44:24, ׳רוֺקַע הָא Psalm 136:6.
Pi`el Imperfect masculine singular suffix בַּזָּהָב יְרַקְּעֶנּוּ Isaiah 40:19 a goldsmith with gold over-layeth it (literally beateth it out3masculine plural וַיְרַקְּעוּ אֶתֿ מַּחֵי הַזָּהָב Exodus 39:3 they beat out the plates of gold; suffix וַיְרִקְּעוּם צִמּוּי Numbers 17:4 they beat them out as plating.
Pu`al Participle כֶּסֶף מְרֻקָּ֑ע Jeremiah 10:9 silver beaten out.
Hiph`il Imperfect masculine singular תַּרְקִיעַ עִמּוֺ לִשְׁחָקִים Job 37:18 canst thou make with (= like) him a spreading for clouds (spread out clouds; cf, רָקִיעַ)?