Run for a second:
yes "some text" > testfile.txt
Reduce size:
truncate -s 50M testfile.txt
Create 50M file using fallocate command
fallocate -l 52428800 testfile.txt
To be more exact using inline arithmetic:
fallocate -l $((50*1024*1024)) > testfile.txt
Using head command
head -c 50MB /dev/urandom > testfile.txt head -c 50MB /dev/zero > testfile.txt
Using dd command
dd if=/dev/urandom of=testfile.txt bs=50MB count=1
Source: https://www.ostechnix.com/create-files-certain-size-linux/