diff --git a/lua/wiremux/backend/tmux/action.lua b/lua/wiremux/backend/tmux/action.lua
index 92e0276..4b6e08a 100644
--- a/lua/wiremux/backend/tmux/action.lua
+++ b/lua/wiremux/backend/tmux/action.lua
@@ -103,4 +103,16 @@ function M.resize_pane_zoom(pane_id)
 	return cmd
 end
 
+---@param direction "horizontal"|"vertical"
+---@param size string
+---@param pane_id? string
+---@return string[]
+function M.resize_pane(direction, size, pane_id)
+	local cmd = { "resize-pane", direction == "horizontal" and "-x" or "-y", size }
+	if pane_id then
+		vim.list_extend(cmd, { "-t", pane_id })
+	end
+	return cmd
+end
+
 return M
diff --git a/lua/wiremux/backend/tmux/operation.lua b/lua/wiremux/backend/tmux/operation.lua
index 7b8a6e8..e187343 100644
--- a/lua/wiremux/backend/tmux/operation.lua
+++ b/lua/wiremux/backend/tmux/operation.lua
@@ -151,10 +151,15 @@ function M.create(target_name, def, st)
 		table.insert(cmds, action.new_window(window_name, not use_shell and cmd or nil))
 		table.insert(cmds, query.window_id())
 	else
+		local split = def.split or "horizontal"
 		table.insert(
 			cmds,
-			action.split_pane(def.split or "horizontal", st.origin_pane_id, not use_shell and cmd or nil)
+			action.split_pane(split, st.origin_pane_id, not use_shell and cmd or nil)
 		)
+		local size = def.size
+		if size then
+			table.insert(cmds, action.resize_pane(split, def.size))
+		end
 		table.insert(cmds, query.pane_id())
 	end
 
diff --git a/lua/wiremux/config.lua b/lua/wiremux/config.lua
index 2dfebd8..0800e35 100644
--- a/lua/wiremux/config.lua
+++ b/lua/wiremux/config.lua
@@ -45,6 +45,7 @@ local M = {}
 ---@field shell? boolean Run command through shell (default: true)
 ---@field label? string|fun(inst: wiremux.Instance, index: number): string Custom display label for picker
 ---@field title? string Custom tmux window / zellij tab name
+---@field size? string Custom tmux pane size
 ---@field startup_timeout? number Max milliseconds to wait for TUI to render before sending (default: 3500)
 
 local defaults = {