The UCSC Kent Utilities fetchChromSizes tool can be used here.

If you just need sizes of nuclear chromosomes (i.e., no pseudo-chromosomes):

$ fetchChromSizes hg38 | grep -v '_*_' > hg38.chromSizes

If you need a BED file for set operations:

$ fetchChromSizes hg38 | grep -v '_*_' | awk -v FS="\t" -v OFS="\t" '{ print $1, "0", $2 }' | sort-bed - > hg38.bed

Replace hg38 with your assembly of interest.