URL: http://dwm.suckless.org/patches/historical/save_floats
This patch saves size and position of every floating window before it is forced
into tiled mode. If the window is made floating again, the old dimensions will
be restored.

diff -r 10e232f9ace7 dwm.c
--- a/dwm.c	Sun Mar 25 17:49:35 2012 +0200
+++ b/dwm.c	Fri Apr 06 08:16:21 2012 +0200
@@ -86,6 +86,7 @@
 	char name[256];
 	float mina, maxa;
 	int x, y, w, h;
+	int sfx, sfy, sfw, sfh; /* stored float geometry, used on mode revert */
 	int oldx, oldy, oldw, oldh;
 	int basew, baseh, incw, inch, maxw, maxh, minw, minh;
 	int bw, oldbw;
@@ -1153,6 +1154,10 @@
 	updatewindowtype(c);
 	updatesizehints(c);
 	updatewmhints(c);
+	c->sfx = c->x;
+	c->sfy = c->y;
+	c->sfw = c->w;
+	c->sfh = c->h;
 	XSelectInput(dpy, w, EnterWindowMask|FocusChangeMask|PropertyChangeMask|StructureNotifyMask);
 	grabbuttons(c, False);
 	if(!c->isfloating)
@@ -1760,10 +1765,18 @@
 		return;
 	selmon->sel->isfloating = !selmon->sel->isfloating || selmon->sel->isfixed;
 	if(selmon->sel->isfloating)
-		resize(selmon->sel, selmon->sel->x, selmon->sel->y,
-		       selmon->sel->w, selmon->sel->h, False);
+		/*restore last known float dimensions*/
+		resize(selmon->sel, selmon->sel->sfx, selmon->sel->sfy,
+		       selmon->sel->sfw, selmon->sel->sfh, False);
 	else if(selmon->sel->isfullscreen)
 		setfullscreen(selmon->sel, False);
+	else {
+		/*save last known float dimensions*/
+		selmon->sel->sfx = selmon->sel->x;
+		selmon->sel->sfy = selmon->sel->y;
+		selmon->sel->sfw = selmon->sel->w;
+		selmon->sel->sfh = selmon->sel->h;
+	}
 	arrange(selmon);
 }