diff --git a/pkg/datasizes/size.go b/pkg/datasizes/size.go
new file mode 100644
index 0000000000..3394669f4b
--- /dev/null
+++ b/pkg/datasizes/size.go
@@ -0,0 +1,72 @@
+package datasizes
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+)
+
+// Size is a wrapper around uint64 with support for reading from string
+// yaml/toml, so {"size": 123}, {"size": "1234"}, {"size": "1 GiB"} are
+// all supported
+type Size uint64
+
+// Uint64 returns the size as uint64. This is a convenience functions,
+// it is strictly equivalent to uint64(Size(1))
+func (si Size) Uint64() uint64 {
+	return uint64(si)
+}
+
+func (si *Size) UnmarshalTOML(data interface{}) error {
+	i, err := decodeSize(data)
+	if err != nil {
+		return fmt.Errorf("error decoding TOML size: %w", err)
+	}
+	*si = Size(i)
+	return nil
+}
+
+func (si *Size) UnmarshalJSON(data []byte) error {
+	dec := json.NewDecoder(bytes.NewBuffer(data))
+	dec.UseNumber()
+
+	var v interface{}
+	if err := dec.Decode(&v); err != nil {
+		return err
+	}
+	i, err := decodeSize(v)
+	if err != nil {
+		// if only we could do better here and include e.g. the field
+		// name where this happend but encoding/json does not
+		// support this, c.f. https://github.com/golang/go/issues/58655
+		return fmt.Errorf("error decoding size: %w", err)
+	}
+	*si = Size(i)
+	return nil
+}
+
+// decodeSize takes an integer or string representing a data size (with a data
+// suffix) and returns the uint64 representation.
+func decodeSize(size any) (uint64, error) {
+	switch s := size.(type) {
+	case string:
+		return Parse(s)
+	case json.Number:
+		i, err := s.Int64()
+		if i < 0 {
+			return 0, fmt.Errorf("cannot be negative")
+		}
+		return uint64(i), err
+	case int64:
+		if s < 0 {
+			return 0, fmt.Errorf("cannot be negative")
+		}
+		return uint64(s), nil
+	case uint64:
+		return s, nil
+	case float64, float32:
+		return 0, fmt.Errorf("cannot be float")
+	default:
+		return 0, fmt.Errorf("failed to convert value \"%v\" to number", size)
+	}
+}
diff --git a/pkg/datasizes/size_test.go b/pkg/datasizes/size_test.go
new file mode 100644
index 0000000000..30b08db50e
--- /dev/null
+++ b/pkg/datasizes/size_test.go
@@ -0,0 +1,125 @@
+package datasizes_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/BurntSushi/toml"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/osbuild/images/pkg/datasizes"
+)
+
+func TestSizeUnmarshalTOMLUnhappy(t *testing.T) {
+	cases := []struct {
+		name  string
+		input string
+		err   string
+	}{
+		{
+			name:  "wrong datatype/bool",
+			input: `size = true`,
+			err:   `toml: line 1 (last key "size"): error decoding TOML size: failed to convert value "true" to number`,
+		},
+		{
+			name:  "wrong datatype/float",
+			input: `size = 3.14`,
+			err:   `toml: line 1 (last key "size"): error decoding TOML size: cannot be float`,
+		},
+		{
+			name:  "wrong unit",
+			input: `size = "20 KG"`,
+			err:   `toml: line 1 (last key "size"): error decoding TOML size: unknown data size units in string: 20 KG`,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var v struct {
+				Size datasizes.Size `toml:"size"`
+			}
+			err := toml.Unmarshal([]byte(tc.input), &v)
+			assert.EqualError(t, err, tc.err, tc.input)
+		})
+	}
+}
+
+func TestSizeUnmarshalJSONUnhappy(t *testing.T) {
+	cases := []struct {
+		name  string
+		input string
+		err   string
+	}{
+		{
+			name:  "misize nor string nor int",
+			input: `{"size": true}`,
+			err:   `error decoding size: failed to convert value "true" to number`,
+		},
+		{
+			name:  "wrong datatype/float",
+			input: `{"size": 3.14}`,
+			err:   `error decoding size: strconv.ParseInt: parsing "3.14": invalid syntax`,
+		},
+		{
+			name:  "misize not parseable",
+			input: `{"size": "20 KG"}`,
+			err:   `error decoding size: unknown data size units in string: 20 KG`,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var v struct {
+				Size datasizes.Size `json:"size"`
+			}
+			err := json.Unmarshal([]byte(tc.input), &v)
+			assert.EqualError(t, err, tc.err, tc.input)
+		})
+	}
+}
+
+func TestSizeUnmarshalHappy(t *testing.T) {
+	cases := []struct {
+		name      string
+		inputJSON string
+		inputTOML string
+		expected  datasizes.Size
+	}{
+		{
+			name:      "int",
+			inputJSON: `{"size": 1234}`,
+			inputTOML: `size = 1234`,
+			expected:  1234,
+		},
+		{
+			name:      "str",
+			inputJSON: `{"size": "1234"}`,
+			inputTOML: `size = "1234"`,
+			expected:  1234,
+		},
+		{
+			name:      "str/with-unit",
+			inputJSON: `{"size": "1234 MiB"}`,
+			inputTOML: `size = "1234 MiB"`,
+			expected:  1234 * datasizes.MiB,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var v struct {
+				Size datasizes.Size `json:"size" toml:"size"`
+			}
+			err := toml.Unmarshal([]byte(tc.inputTOML), &v)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expected, v.Size, tc.inputTOML)
+			err = json.Unmarshal([]byte(tc.inputJSON), &v)
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expected, v.Size, tc.inputJSON)
+		})
+	}
+}
+
+func TestSizeUint64(t *testing.T) {
+	assert.Equal(t, datasizes.Size(1234).Uint64(), uint64(1234))
+}