You can find clustering configuration in template file.

in case of template bootstrap2-responsive can be found in:

templates\bootstrap2-responsive\components\head.php, line around 110:

In latest script version file: templates\{your-template}\widgets\head.php

// Cluster config start //
clusterConfig = {
radius: 60,
// This style will be used for clusters with more than 2 markers
2: {
content: "<div class='cluster cluster-1'>CLUSTER_COUNT</div>",
width: 53,
height: 52
},
// This style will be used for clusters with more than 5 markers
5: {
content: "<div class='cluster cluster-1'>CLUSTER_COUNT</div>",
width: 53,
height: 52
},
// This style will be used for clusters with more than 20 markers
20: {
content: "<div class='cluster cluster-2'>CLUSTER_COUNT</div>",
width: 56,
height: 55
},
// This style will be used for clusters with more than 50 markers
50: {
content: "<div class='cluster cluster-3'>CLUSTER_COUNT</div>",
width: 66,
height: 65
},

Or in case of realocation:

templates\realocation\assets\js\map_custom.js