UPDATED: this is reproducible in a non-MDI scenario is well.

Sub New()
    InitializeComponent()
    Me.Size = New Size(500, 500)
End Sub


To reproduce:

1. Add two RadForms - a parent and a child.
2. Set the MinimumSize property of the child form at design time.
3. Use the following code snippet:

public partial class ParentForm : RadForm
{
    public ParentForm()
    {
        InitializeComponent();

        this.IsMdiContainer = true;

        ChildForm form = new ChildForm();
        form.Text = "MDI Child 1";
        form.MdiParent = this;
        form.Size = new System.Drawing.Size(600, 600);
        form.Show();
    }
}

public partial class ChildForm : Telerik.WinControls.UI.RadForm
{
    public ChildForm()
    {
        InitializeComponent();
        this.SizeChanged += ChildForm_SizeChanged;
    }
    
    private void ChildForm_SizeChanged(object sender, EventArgs e)
    {
        this.Text = "ChildForm " + this.Size.Width + " x " + this.Size.Height;
    }
}

When you run the application with version 2015.2 623, you will notice that the child form is loaded with its MinimumSize. However, in the previous version it is loaded with the expected specified Size(600, 600).

Workaround: use ShapedForm with titlebar. An alternative solution is to set the RadForm.Size property in the Shown event.