diff --git a/beacon_chain/conf.nim b/beacon_chain/conf.nim
index fe40352b4..5263f5808 100644
--- a/beacon_chain/conf.nim
+++ b/beacon_chain/conf.nim
@@ -451,9 +451,12 @@ type
         name: "rest-max-body-size" .}: Natural
 
       restMaxRequestHeadersSize* {.
-        defaultValue: 64
+        defaultValue: 128
         desc: "Maximum size of REST request headers (kilobytes)"
         name: "rest-max-headers-size" .}: Natural
+        ## NOTE: If you going to adjust this value please check value
+        ## ``ClientMaximumValidatorIds`` and comments in
+        ## `spec/eth2_apis/rest_types.nim`. This values depend on each other.
 
       keymanagerEnabled* {.
         desc: "Enable the REST keymanager API"
diff --git a/beacon_chain/rpc/rest_beacon_api.nim b/beacon_chain/rpc/rest_beacon_api.nim
index 56aa8616e..ed279d01e 100644
--- a/beacon_chain/rpc/rest_beacon_api.nim
+++ b/beacon_chain/rpc/rest_beacon_api.nim
@@ -254,7 +254,7 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
           return RestApiResponse.jsonError(Http400,
                                            InvalidValidatorIdValueError)
         let ires = id.get()
-        if len(ires) > MaximumValidatorIds:
+        if len(ires) > ServerMaximumValidatorIds:
           return RestApiResponse.jsonError(Http400,
                                            MaximumNumberOfValidatorIdsError)
         ires
@@ -445,7 +445,7 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
           return RestApiResponse.jsonError(Http400,
                                            InvalidValidatorIdValueError)
         let ires = id.get()
-        if len(ires) > MaximumValidatorIds:
+        if len(ires) > ServerMaximumValidatorIds:
           return RestApiResponse.jsonError(Http400,
                                            MaximumNumberOfValidatorIdsError)
         ires
diff --git a/beacon_chain/spec/eth2_apis/rest_types.nim b/beacon_chain/spec/eth2_apis/rest_types.nim
index d105bef3a..411ee9f0f 100644
--- a/beacon_chain/spec/eth2_apis/rest_types.nim
+++ b/beacon_chain/spec/eth2_apis/rest_types.nim
@@ -27,11 +27,35 @@ export forks, phase0, altair, bellatrix, capella, bellatrix_mev
 const
   # https://github.com/ethereum/eth2.0-APIs/blob/master/apis/beacon/states/validator_balances.yaml#L17
   # https://github.com/ethereum/eth2.0-APIs/blob/master/apis/beacon/states/validators.yaml#L17
-  MaximumValidatorIds* = 16384
+  # Maximum number of validators that can be served by the REST server in one
+  # request, if the number of validator exceeds this value REST server
+  # will return HTTP error 400.
+  ServerMaximumValidatorIds* = 16384
+
+  # Maximum number of validators that can be sent in single request by
+  # validator client (VC).
+  # NOTE: This value depend on beacon node's `rest-max-headers-size`
+  # configuration option.
+  #
+  # Size of public key in HTTP request could be calculated by formula -
+  # bytes48 * 2 + len("0x") + len(",") = 99 bytes.
+  # So 1024 keys will occupy 101,376 bytes. Default value for HTTP headers size
+  # is 128Kb = 131,072 bytes.
+  ClientMaximumValidatorIds* = 1024
+
+  # https://github.com/ethereum/beacon-APIs/blob/master/apis/validator/duties/attester.yaml#L32
+  # https://github.com/ethereum/beacon-APIs/blob/master/apis/validator/duties/sync.yaml#L16
+  # Maximum number of validator ids sent with validator client's duties
+  # requests. Validator ids are sent in decimal encoding with comma, so
+  # number of ids should not exceed beacon node's `rest-max-body-size`.
+  DutiesMaximumValidatorIds* = 16384
 
 const
   preferSSZ* = "application/octet-stream,application/json;q=0.9"
 
+static:
+  doAssert(ClientMaximumValidatorIds <= ServerMaximumValidatorIds)
+
 type
   EventTopic* {.pure.} = enum
     Head, Block, Attestation, VoluntaryExit, FinalizedCheckpoint, ChainReorg,
diff --git a/beacon_chain/validator_client/duties_service.nim b/beacon_chain/validator_client/duties_service.nim
index 0c6516550..aae81356e 100644
--- a/beacon_chain/validator_client/duties_service.nim
+++ b/beacon_chain/validator_client/duties_service.nim
@@ -51,7 +51,7 @@ proc pollForValidatorIndices*(vc: ValidatorClientRef) {.async.} =
   var offset = 0
 
   while offset < len(validatorIdents):
-    let arraySize = min(MaximumValidatorIds, len(validatorIdents))
+    let arraySize = min(ClientMaximumValidatorIds, len(validatorIdents))
 
     let idents =
       block:
@@ -119,7 +119,9 @@ proc pollForAttesterDuties*(vc: ValidatorClientRef,
 
   var offset = 0
   while offset < len(validatorIndices):
-    let arraySize = min(MaximumValidatorIds, len(validatorIndices))
+    let arraySize = min(DutiesMaximumValidatorIds, len(validatorIndices))
+    # We use `DutiesMaximumValidatorIds` here because validator ids are sent
+    # in HTTP request body and NOT in HTTP request headers.
     let indices =
       block:
         var res = newSeq[ValidatorIndex](arraySize)
@@ -255,7 +257,9 @@ proc pollForSyncCommitteeDuties*(vc: ValidatorClientRef,
 
   while offset < len(validatorIndices):
     let
-      arraySize = min(MaximumValidatorIds, remainingItems)
+      arraySize = min(DutiesMaximumValidatorIds, remainingItems)
+      # We use `DutiesMaximumValidatorIds` here because validator ids are sent
+      # in HTTP request body and NOT in HTTP request headers.
       indices = validatorIndices[offset ..< (offset + arraySize)]
 
       res =