how to set gridview row height

Lot of ways there
Mention the height value for RowStyle(& AlternateRowStyle) in your HTML source

<rowstyle height="50" />

You can do that same in code-behind

GridView1.RowStyle.Height = 50;

---

But my suggestion is use CSS(Best way)

.RowStyle {
  height: 50px;
}
.AlternateRowStyle {
  height: 50px;
}

HTML Source

<asp:gridview id="GridView1" runat="server" xmlns:asp="#unknown">
    RowStyle-CssClass="RowStyle"  
    AlternatingRowStyle-CssClass="AlternateRowStyle">  
</asp:gridview>

OR

<asp:gridview id="GridView1" runat="server" xmlns:asp="#unknown">
<rowstyle cssclass="RowStyle" />
<alternatingrowstyle cssclass="AlternateRowStyle" />
</asp:gridview>