ATAC seq : Plotting insert sizes

My tutorial link

library(GenomicAlignments)

atacReads <- readGAlignmentPairs("MACS2_peak_call/ALL_ATAC_data/HSC1.bam", param = ScanBamParam(mapqFilter = 1, 
            flag = scanBamFlag(isPaired = TRUE, isProperPair = TRUE), what = c("qname", 
        "mapq", "isize"), which = GRanges("chr20", IRanges(1, 63025520))))
# length(atacReads)
atacReads


atacReads_read1 <- GenomicAlignments::first(atacReads)
insertSizes <- abs(elementMetadata(atacReads_read1)$isize)
head(insertSizes)


library(magrittr)
library(dplyr)
library(ggplot2)
fragLenPlot <- table(insertSizes) %>% data.frame %>% dplyr::rename(InsertSize = insertSizes, 
    Count = Freq) %>% mutate(InsertSize = as.numeric(as.vector(InsertSize)), 
 Count = as.numeric(as.vector(Count))) %>% ggplot(aes(x = InsertSize, y = Count)) + 
  geom_line()

fragLenPlot + theme_bw()

fragLenPlot + scale_y_continuous(trans = "log2") + theme_bw()

How to i specify the Grange to span the total chromosome. Now I use only "chr20" as shown in the example.