Siman 396
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א שְׁבוּ אִישׁ תַּחְתָּיו (שְׁמוֹת טז, כט), מִכָּאן שֶׁכָּל אָדָם יֵשׁ לוֹ ד' אַמּוֹת בְּכָל מָקוֹם, אֲפִלּוּ יָצָא חוּץ לַתְּחוּם. וּמוֹדְדִים לְכָל אָדָם בָּאַמָּה שֶׁלּוֹ, וְאִם הָיָה נַנָּס בְּאֵיבָרָיו נוֹתְנִים לוֹ אַרְבַּע אַמּוֹת בֵּינוֹנִיּוֹת שֶׁל כָּל אָדָם, שֶׁכָּל אַחַת מֵהֶן שִׁשָּׁה טְפָחִים. הַגָּה: וְאֵלּוּ ד' אַמּוֹת מוֹדְדִין לוֹ מְרֻוָּחוֹת, וְהוּא בְּאֶמְצָעָן. (טוּר). וְיֵשׁ אוֹמְרִים דְּמֻתָּר לְהַלֵּךְ אַרְבַּע אַמּוֹת לְכָל צַד, אֲבָל לְטַלְטֵל אֵין לוֹ רַק אַרְבַּע אַמּוֹת עִם אֲלַכְסוֹנָן, כְּמוֹ שֶׁנִּתְבָּאֵר לְעֵיל סִימָן שמ"ט. (טוּר בְּשֵׁם הרז"ה פי"ב בְּשֵׁם רַשְׁבָּ"א).
1 "Each man has under him" (_Exodus 16:29_), from this phrase we learn that each person has four amot (cubits) in every place, even if he went outside of the barrier and we measure for each person his "amah", and if he has short limbs, we give him four average-length amot, each of which is six handbreaths. Rema: And these four amot are measured wide with him in the center (Tur). And there are those who say that he is permitted to walk four amot on each side but carrying, he only has four amot at the diagonal, as was explained before in Siman 349 (Tur in the name of the Raza''h and the Maggid Ch. 12 in the name of the Rashbah.)
ב בְּכָל מָקוֹם שֶׁקָּדַשׁ הַיּוֹם עָלָיו, אִם הוּא מֻקָּף לְדִירָה אֲפִלּוּ אֵין בּוֹ עַתָּה דִּיּוּרִין, חָשׁוּב כֻּלּוֹ כְּד' אַמּוֹת. וְאִם אֵינוֹ מֻקָּף לַדִּירָה, עַד סְאָתַיִם חָשׁוּב כֻּלּוֹ כְּד' אַמּוֹת. וַאֲפִלּוּ שָׁבַת בְּתֵל גָּבוֹהַּ, וּבְקָמָה קְצוּרָה וְשִׁבֳּלִים מַקִּיפוֹת אוֹתָהּ, חֲשִׁיבֵי כְּד' אַמּוֹת עַד סְאָתַיִם.
2 In any place that he finds himself when Shabbat begins, if he is in an enclosed space for dwelling, even if there are no inhabitants, all of it is considered like four amot, and if it is not an enclosed space for dwelling, until it contains two seahs, all of it is considered like four amot, and even he is on a tall hill, an harvested area with stalks and barley surrounding it, it is considered like four amot until it contains two seahs.