diff --git a/cmd-new-session.c b/cmd-new-session.c
index 7af67a11..e809de24 100644
--- a/cmd-new-session.c
+++ b/cmd-new-session.c
@@ -202,17 +202,29 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 		sy = 24;
 	}
 	if ((is_control || detached) && args_has(args, 'x')) {
-		sx = strtonum(args_get(args, 'x'), 1, USHRT_MAX, &errstr);
-		if (errstr != NULL) {
-			cmdq_error(item, "width %s", errstr);
-			goto error;
+		tmp = args_get(args, 'x');
+		if (strcmp(tmp, "-") == 0) {
+			if (c != NULL)
+				sx = c->tty.sx;
+		} else {
+			sx = strtonum(tmp, 1, USHRT_MAX, &errstr);
+			if (errstr != NULL) {
+				cmdq_error(item, "width %s", errstr);
+				goto error;
+			}
 		}
 	}
 	if ((is_control || detached) && args_has(args, 'y')) {
-		sy = strtonum(args_get(args, 'y'), 1, USHRT_MAX, &errstr);
-		if (errstr != NULL) {
-			cmdq_error(item, "height %s", errstr);
-			goto error;
+		tmp = args_get(args, 'y');
+		if (strcmp(tmp, "-") == 0) {
+			if (c != NULL)
+				sy = c->tty.sy;
+		} else {
+			sy = strtonum(tmp, 1, USHRT_MAX, &errstr);
+			if (errstr != NULL) {
+				cmdq_error(item, "height %s", errstr);
+				goto error;
+			}
 		}
 	}
 	if (sx == 0)
diff --git a/tmux.1 b/tmux.1
index ac5e9b44..329279fd 100644
--- a/tmux.1
+++ b/tmux.1
@@ -851,6 +851,8 @@ the initial size is 80 x 24;
 and
 .Fl y
 can be used to specify a different size.
+.Ql -
+uses the size of the current client if any.
 .Pp
 If run from a terminal, any
 .Xr termios 4