How can I make a 16x16 led pannel without losing the brightness intensity of each single led?
The most common solution to driving large numbers of LEDs at (approximately) equal brightness is to use constant current drivers.  A common size driver is 250W.  If you used that size, you would set up about 5 series circuits, each consisting of about 50 LEDs.  You would then choose a driver with 700mA current and 360V maximum voltage.  This is a common voltage/current range from vendors such as Meanwell (e.g. HLG-240H) and many others.  Dimming is usually included.  Alternatively, if you wanted lower voltage, you could put two parallel strings of 25 LEDs and use 1400mA.  

It should go without saying, but take care that your wiring is properly isolated.  High voltages keep the current reasonable, but they can be very dangerous if you come into contact with them.