You could combine the suggestions so far into a script like so (the BAM file should be name sorted):

samtools view -h name_sorted.bam  | awk -f filter.awk | samtools view -Sb - | bedtools bamtofastq -i - -fq read1.fq
samtools view -h name_sorted.bam  | awk -f filter.awk | samtools view -Sb - | bedtools bamtofastq -i - -fq2 read2.fq

The filter.awk file is

# SIZE is the minimum template lenght
BEGIN { FS="\t"; SIZE=100; S2=SIZE*SIZE }

# Write the headers 
/^@/ { print $0; next }

# Print only long entries
{ if ($9*$9 > S2) print $0}

Filtering on Template length for paired-end reads (Your bam file should be name sorted)

samtools view sample.bam | \
  awk '
    BEGIN {FS='\t'}
    { if ($9 > 100 || $9 < 500) print $0}
  ' | \
  grep -v ^@ | \
  awk 'NR%2==1 {print "@"$1"\n"$10"\n+\n"$11}' > ./sample_1.fastq

samtools view sample.bam | \
  awk '
    BEGIN {FS='\t'}
    { if ($9 > 100 || $9 < 500) print $0}
  ' | \
  grep -v ^@ | \
  awk 'NR%2==0 {print "@"$1"\n"$10"\n+\n"$11}' > ./sample_2.fastq

Filtering on Template length for single-end reads

samtools view sample.bam | \
  awk '
    BEGIN {FS='\t'}
    { if ($9 > 100 || $9 < 500) print $0}
  ' | \
  grep -v ^@ | \
  awk '{print "@"$1"\n"$10"\n+\n"$11}' > ./sample.fastq

Reference here

You can try something like this. In order to print both reads for a read-pair you will have to use 'OR' operator as shown. I have used a threshold of 100 (highlighted in line 2) for TLEN.

samtools view input.bam | awk 'BEGIN {FS="\t"} {if ($9 > 100 || $9 < -100) print "@" $1 "\n" $10 "\n+\n" $11}' > input.fq
                                                   ^_____________________^