Suitable microcontroller??????????

I'm working on the LED costume project right now. This requires approximately 25 meters of ws2815 led strip (60 leds/per meter). I am preparing a number of projects lasting up to 8 min. As a beginner, I bought arduino UNO R3 and then realized it is not enough. I found on the Internet that Arduino Due is a microcontroller with the biggest memory. Will it be enough to run my projects? Or is it more efficient to use multiple UNO R3 at the same time?

You would need to give us much better descriptions about what you want the LEDS to do, if you control clusters or single LEDS, number of LEDs per Cluster (controled together) Number of clusters.
so for hints I give you hints what to look for
Although UNO is simply massive while its ok for testing or developement, in practical project nano is better (pro-miny is not for beginners).
if there are enough outputs on Mega it is better then using multiple controllers. If not then I would recommend one master and multiple slave layout. (not a starter think)
if your strip uses 12V I recommend ULN2003.
If memory keeps being a problem you can use cloud storage with esp8266 and a bit of PHP...

How are you planning to power 25 meters of LED strip? It seems like you might have to drag an automobile battery around with your costume to have sufficient current for a sufficiently long time unless you are going to really restrict the patterns and/or the colors that you use.

I agree with vaj, power will be the more difficult challenge than chosing a suitable arduino for this project. 1,500 leds could require a maximum of 75A! But that is if all 1,500 leds are at full brightness white at the same time. Other colours, lower brightness and patterns where only some leds are lit at any instant will require far less current, so design your patterns and effects carefully with a power budget in mind. Even when all 1,500 leds are off, the strip could require 3A+, becase the quiescent current of each led is over 2mA. If you use the FastLED library, you can set a maximum current limit, chosen based on your power supply, and this will automatically lower the brightness of the strip if the colours and patterns in use would exceed that limit.

For a suitable Arduino, take a look at those offered by Adafruit, such as the GEMMA M0, Trinket M0, Itsy bitsy M0 or Feather M0. Note that these are all 3.3V devices and ws2815 requires a data signal of at least 8.5V (0.7 x Vdd). You will need to use a voltage shifter. Do not choose the voltage shifter modules commonly sold on eBay etc, they are not suitable for ws28xx leds.

ws2815 data sheet

Maybe its better to make sketch first and then depending on its size, choose a suitable microcontroller???

Maybe its better to make sketch first and then depending on its size, choose a suitable microcontroller???

A reasonable strategy. It's probably sufficient to declare the appropriate number of LEDs you're going to use - that will show you how much RAM is required. The other concern is how many pins you need.