#PHP, 306 292 281 282 281 275 270 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if(",">$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if(9<$c)$r.=$r;if(w<$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.

PHP, 306 292 281 282 281 275 270 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if(",">$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if(9<$c)$r.=$r;if(w<$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.

#PHP, 306 292 281 282 281 275 273 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if(",">$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if("="==$c)$r.=$r;if(w<$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.

#PHP, 306 292 281 282 281 275 270 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if(",">$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if(9<$c)$r.=$r;if(w<$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.

#PHP, 306 292 281 282 281 275 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if("+"==$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if("="==$c)$r.=$r;if(x==$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.

#PHP, 306 292 281 282 281 275 273 bytes

$r=str_pad("",2*$n=$argv[2],($c=$argv[1])." ")."\n";if(",">$c)$r=($m=str_repeat(str_pad($c,$n," ",0)."\n",$n/2))."$r$m";if("="==$c)$r.=$r;if(w<$c)for($r=$c;$i<$n;$r[$i*(2+$n)]=$r[++$i*$n-1]=x)$r[$i*$n+$i+$n]="\n";if("/"==$c)for($r=$s="";$i++<$n;$s.=" ")$r="$s/\n$r";echo$r;

Ooh this is bulky ... needs further golfing.
I could save 4 bytes with physical linebreaks or one byte with putting "\n" into a variable.