diff --git a/internal/util/bytesize.go b/internal/util/bytesize.go
index 9403af8..091d3c5 100644
--- a/internal/util/bytesize.go
+++ b/internal/util/bytesize.go
@@ -4,15 +4,21 @@ import (
 	"fmt"
 	"math"
 	"strconv"
+	"strings"
 
 	"github.com/dustin/go-humanize"
 )
 
 // ByteSize is a size in bytes, configured as a string such as "32KiB", "1MB"
 // or "1.5 GB" (see ParseByteSize). KB, MB, ... are powers of 1000 and KiB,
-// MiB, ... powers of 1024; a bare number is bytes.
+// MiB, ... powers of 1024; a bare number is bytes. ByteSizeNoLimit (-1) means
+// no limit; no other size is negative.
 type ByteSize int64
 
+// ByteSizeNoLimit is the size that means no limit, written "-1". A setting
+// for which no limit is unsafe rejects it.
+const ByteSizeNoLimit ByteSize = -1
+
 // byteUnits are the units String may use, largest first.
 var byteUnits = []struct {
 	name string
@@ -27,8 +33,16 @@ var byteUnits = []struct {
 }
 
 // ParseByteSize parses a size such as "32KiB", "1 MB" or "4096". Unit names
-// are case-insensitive.
+// are case-insensitive. "-1" (or "-1B") is ByteSizeNoLimit; any other
+// negative size is an error.
 func ParseByteSize(s string) (ByteSize, error) {
+	trimmed := strings.TrimSpace(s)
+	if trimmed == "-1" || strings.EqualFold(trimmed, "-1B") {
+		return ByteSizeNoLimit, nil
+	}
+	if strings.HasPrefix(trimmed, "-") {
+		return 0, fmt.Errorf("byte size %q is negative; only -1 (no limit) is allowed", s)
+	}
 	n, err := humanize.ParseBytes(s)
 	if err != nil {
 		return 0, err
@@ -41,7 +55,11 @@ func ParseByteSize(s string) (ByteSize, error) {
 
 // String formats b exactly, in the largest unit that divides it (e.g. "32KiB",
 // "1MB"), or in bytes ("1500B"), so it parses back to the same value.
+// ByteSizeNoLimit is "-1".
 func (b ByteSize) String() string {
+	if b == ByteSizeNoLimit {
+		return "-1"
+	}
 	if b != 0 {
 		for _, u := range byteUnits {
 			if int64(b)%u.size == 0 {
diff --git a/internal/util/bytesize_test.go b/internal/util/bytesize_test.go
index 3b118c6..0861f7e 100644
--- a/internal/util/bytesize_test.go
+++ b/internal/util/bytesize_test.go
@@ -1,6 +1,9 @@
 package util
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestByteSizeString(t *testing.T) {
 	tests := []struct {
@@ -25,3 +28,23 @@ func TestByteSizeString(t *testing.T) {
 		}
 	}
 }
+
+// -1 is ByteSizeNoLimit, parsed and formatted as "-1"; no other size may be
+// negative.
+func TestByteSizeNoLimit(t *testing.T) {
+	for _, text := range []string{"-1", "-1B", "-1b", " -1 "} {
+		size, err := ParseByteSize(text)
+		if err != nil || size != ByteSizeNoLimit {
+			t.Errorf("ParseByteSize(%q) = %d, %v; want ByteSizeNoLimit", text, size, err)
+		}
+	}
+	if got := ByteSizeNoLimit.String(); got != "-1" {
+		t.Errorf("ByteSizeNoLimit.String() = %q; want -1", got)
+	}
+	for _, text := range []string{"-2", "-1KB", "-0.5", "-1KiB", "-100"} {
+		size, err := ParseByteSize(text)
+		if err == nil || !strings.Contains(err.Error(), "only -1 (no limit)") {
+			t.Errorf("ParseByteSize(%q) = %d, %v; want a negative size error", text, size, err)
+		}
+	}
+}